U.S. patent application number 10/885339 was filed with the patent office on 2005-11-17 for switchable optical components.
Invention is credited to Domash, Lawrence H., Little, Brent.
Application Number | 20050254752 10/885339 |
Document ID | / |
Family ID | 25172177 |
Filed Date | 2005-11-17 |
United States Patent
Application |
20050254752 |
Kind Code |
A1 |
Domash, Lawrence H. ; et
al. |
November 17, 2005 |
Switchable optical components
Abstract
This invention relates to a number of components, devices and
networks involving integrated optics and/or half coupler
technology, all of which involve the use of electronically
switchable Bragg grating devices and device geometries realized
using holographic polymer/dispersed liquid crystal materials. Most
of the components and devices are particularly adapted for use in
wavelength division multiplexing (WDM) systems and in particular
for use in switchable add/drop filtering (SADF) and wavelength
selective crossconnect. Attenuators, outcouplers and a variety of
other devices are also provided.
Inventors: |
Domash, Lawrence H.;
(Conway, MA) ; Little, Brent; (Boston,
MA) |
Correspondence
Address: |
JERRY RICHARD POTTS
3248 VIA RIBERA
ESCONDIDO
CA
92029
US
|
Family ID: |
25172177 |
Appl. No.: |
10/885339 |
Filed: |
July 6, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10885339 |
Jul 6, 2004 |
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10107593 |
Mar 26, 2002 |
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6771857 |
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10885339 |
Jul 6, 2004 |
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08797950 |
Feb 12, 1997 |
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5937115 |
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Current U.S.
Class: |
385/37 |
Current CPC
Class: |
G02F 1/1334 20130101;
G02F 1/313 20130101; G02B 2006/12107 20130101; G02F 1/13342
20130101; G02F 2201/30 20130101; G02B 2006/12145 20130101 |
Class at
Publication: |
385/037 |
International
Class: |
G02B 006/34 |
Claims
1. A component for selectively reconfiguring an optical signal on a
guided wave optical path including: at least on ESBG in optical
contact with said path; and electrodes for selectively applying at
least a first and a second voltage across each said ESBG, there
being no change in the optical signal for said path when said first
voltage is across a said ESBG and there being a selected change in
the optical signal on said path when said second voltage is across
the ESBG.
2. A component as claimed in claim 1 wherein said optical signal is
a multichannel WDM signal, and wherein a selected channel is
dropped from said path when said second voltage is across the
ESBG.
3. A component as claimed in claim 2 wherein said channel is
dropped by being retroreflected back through said path.
4. A component as claimed in claim 3 wherein said channel
outcoupled from said path.
5. A component as claimed in claim 1 wherein said optical signal is
a multichannel WDM signal, and including a second guided wave
optical path optically coupled to a said ESBG, at least one
selected channel being transferred between said paths when said
second voltage is across said ESBG.
6. A component as claimed in claim 5 wherein a channel is dropped
from said optical path by being transferred to said second optical
path when said second voltage is across said ESBG.
7. A component as claimed in claim 5 wherein a channel is added to
said path by being transferred to said path from said second path
when said second voltage is across said ESBG.
8. A component as claimed in claim 5 wherein signals having WDM
channels appear on both paths, and wherein a selected channel is
transferred from one path to the other when a second voltage is
across said ESBG.
9. A component as claimed in claim 5 wherein there are a plurality
of ESBGs optically coupled to both paths, and wherein a different
channel is transferred between paths by each ESBG when said second
voltage is thereacross.
10. A component as claimed in claim 5 wherein there is an ESBG in
optical contact with each of said paths, and including at least one
optical path interconnecting said ESBGs.
11. A component as claimed in claim 10 wherein there are two
optical paths interconnecting said ESBGs to form a ring.
12. A component as claimed in claim 11 wherein there is one of said
rings between said paths for each channel to be transferred
therebetween.
13. A component as claimed in claim 11 wherein the said optical
path and second optical path have a first effective index, and
wherein the optical paths for said ring have a second effective
index different than first effective index.
14. A component as claimed in claim 13 wherein the optical paths
for said ring are of a different size than the said optical path
and second optical path.
15. A component as claimed in claim 5 wherein there are a plurality
of ESBGs optically coupled to both paths, and wherein a different
channel is transferred between paths by each ESBG when said second
voltage is thereacross.
16. A component as claimed in claim 5 wherein said optical paths
have different indexes.
17. A component as claimed in claim 16 wherein said optical paths
are of different size.
18. A component as claimed in claim 16 wherein said ESBG has a
grating with an index contrast .DELTA.n, and wherein the difference
in the effective index of the optical paths is
.gtoreq..DELTA.n.
19. A component as claimed in claim 5 wherein said component is
part of an integrated optics device.
20. A component as claimed in claim 19 wherein said optical paths
have different indexes, and wherein said ESBG has a grating with an
index contrast .DELTA.n, and wherein the difference in the
effective index of the optical paths is .gtoreq..DELTA.n.
21. A component as claimed in claim 5 wherein said component is
part of a coupler half device.
22. A component as claimed in claim 21 wherein said component is a
switchable drop filter.
23. A component as claimed in claim 21 wherein said component is a
switchable outcoupler.
24. A component as claimed in claim 21 wherein said component is an
attenuator.
25. A component as claimed in claim 24 wherein said ESBG has a
submicron grating.
26. A component as claimed in claim 5 wherein said ESBG is a
resonator.
27. A component as claimed in claim 26 wherein said component is a
channel drop filter, and including a first drop resonator and a
second reflector resonator spaced from each other in the direction
of travel of said optical signal on a said optical path by an
integer number of wavelengths of a wavelength to be dropped plus a
half wavelength.
28. A component as claimed in claim 27 wherein each resonator is a
multipole resonator formed of resonator sections which are one of
series coupled and parallel coupled.
29. A component as claimed in claim 26 wherein said component is a
guide channel dropping filter, said ESBG being a resonator between
the optical paths.
30. A component as claimed in claim 29 wherein said resonator is a
split resonator having a phase delay section between split
resonator sections.
31. A component as claimed in claim 5 wherein each said optical
path has a core surrounded by cladding, and wherein said ESBG is in
the cladding for both said optical paths.
32. A component as claimed in claim 31 wherein the cladding for
said optical paths overlap, and wherein said ESBG is in the overlap
of said cladding.
33. A component as claimed in claim 31 wherein said ESBG extends
over both said optical paths.
34. A component as claimed in claim 5 wherein sidelobes are
suppressed by apodization.
35. An integrated optics component for selectively reconfiguring an
optical signal including: a substrate; at least one optical
waveguide formed in said substrate, each said waveguide having a
core with cladding therearound; an ESBG in one of said core and
said cladding for a said waveguide; and electrodes for selectively
applying at least a first and a second voltage across said ESBG,
there being no change in the optical signal when applied to the
said waveguide if said first voltage is across the ESBG and there
bing a selected change in the optical if said second signal is
across the ESBG.
36. A component as claimed in claim 35 wherein said optical signal
is a multichannel WDM signal, and wherein a selected channel is
dropped from said waveguide when said second voltage is across the
ESBG.
37. A component as claimed in claim 35 wherein said optical signal
is a multichannel WDM signal, and including a second guided wave
optical waveguide optically coupled to a said ESBG, at least one
selected channel being transferred between said waveguides when
said second voltage is across said ESBG.
38. A component as claimed in claim 37 wherein a channel is dropped
from said optical waveguide by being transferred to said second
optical waveguide when said second voltage is across said ESBG.
39. A component as claimed in claim 37 wherein a channel is added
to said waveguide by being transferred to said waveguide from said
second waveguide when said second voltage is across said ESBG.
40. A component as claimed in claim 37 wherein signals having WDM
channels appear on both waveguides, and wherein a selected channel
is transferred from one waveguide to the other when a second
voltage is across said ESBG.
41. A component as claimed in claim 37 wherein there are a
plurality of ESBGs optically coupled to both waveguides, and
wherein a different channel is transferred between waveguides by
each ESBG when said second voltage is thereacross.
42. A component as claimed in claim 37 wherein there is an ESBG in
optical contact with each of said waveguides, and including at
least one optical waveguide interconnecting said ESBGs.
43. A component as claimed in claim 42 wherein there are two
optical waveguides interconnecting said ESBGs to form a ring.
44. A component as claimed in claim 43 wherein there is one of said
rings between said waveguides for each channel to be transferred
therebetween.
45. A component as claimed in claim 43 wherein the said optical
waveguide and second optical waveguide have a first effective
index, and wherein the optical waveguides for said ring have a
second effective index different than the first effective
index.
46. A component as claimed in claim 45 wherein the optical
waveguides for said ring are of a different size than the said
optical waveguide and second optical waveguide.
47. A component as claimed in claim 37 wherein there are a
plurality of ESBGs optically coupled to both waveguides, and
wherein a different channel is transferred between waveguides by
each ESBG when said second voltage is thereacross.
48. A component as claimed in claim 37 wherein said optical
waveguides have different indexes.
49. A component as claimed in claim 48 wherein said optical
waveguides are of different size.
50. A component as claimed in claim 48 wherein said ESBG has a
grating with an index contrast .DELTA.n, and wherein the difference
in the effective index of the optical waveguides is
.gtoreq..DELTA.n.
51. A component as claimed in claim 37 wherein said ESBG has a
submicron grating.
52. A component as claimed in claim 51 wherein said ESBG is a
resonator.
53. A component as claimed in claim 52 wherein said component is a
channel drop filter, and including a first drop resonator and a
second reflector resonator spaced from each other in the direction
of travel of said optical signal on a said optical waveguide by an
integer number of wavelengths of a wavelength to be dropped, plus a
half wavelength.
54. A component as claimed in claim 51 wherein said component is a
guide channel dropping filter, said ESBG being a resonator between
the channels.
55. A component as claimed in claim 30 wherein said resonator is a
split resonator having a phase delay section between split
resonator sections.
56. A component as claimed in claim 37 wherein each said optical
waveguide has a core surrounded by cladding, and wherein said ESBG
is in the cladding for both said optical waveguides.
57. A component as claimed in claim 56 wherein the cladding for
said optical waveguides overlap, and wherein said ESBG is in the
overlap of said cladding.
58. A component as claimed in claim 56 wherein said ESBG extends
over both said optical waveguides.
59. A component as claimed in claim 37 wherein sidelobes are
suppressed by apodization.
60. A coupler half component for selectively reconfiguring an
optical signal including: an optical fiber having a core with
cladding therearound, the core having an index n.sub.1, the
cladding having an index n.sub.2, and the effective index of the
fiber being n.sub.e, the cladding being at least partially removed
in a selected region; an ESBG mounted to said fiber in said region,
said ESBG having an index n.sub.B when in a first state, where
n.sub.B is substantially equal to n.sub.2; and electrical elements
including electrodes for selectively applying a voltage across said
ESBG, the effect which the ESBG has on light applied to the ESBG
varying as the voltage thereacross changes, thereby changing the
state of the ESBG.
61. A component as claimed in claim 60 wherein light applied to
said optical fiber is substantially unaffected by said ESBG when
the ESBG is in said first state.
62. A component as claimed in claim 61 wherein said ESBG is in said
first state when the mechanism applies substantially no voltage
across to said ESBG.
63. A component as claimed in claim 61 wherein variations in
n.sub.B as a result of voltage changes across the ESBG results in
selective attenuation of light applied to said fiber.
64. A component as claimed in claim 63 wherein said light is
multichannel light at different wavelengths, and wherein said
attenuation is substantially independent of channel.
65. A component as claimed in claim 61 wherein said light is a
multichannel multiwavelength signal, and wherein said ESBG causes
light at one or more selected wavelengths to be coupled through the
ESBG in at least one direction to or from said fiber as the voltage
across of said ESBG is varied from a voltage causing the ESBG to be
in said first state.
66. A component as claimed in claim 61 wherein said ESBG has a
grating period which causes light at one or more selected
wavelengths of a multiwavelength light signal to be reflected back
along said fiber, thereby filtering said one or more selected
wavelengths from the light propagating in said fiber.
67. A component as claimed in claim 61 wherein there are two of
said fibers with the said region of the fibers being adjacent, and
with said ESBG being mounted between the fibers in both said
regions, all light on one of said fibers passing unchanged through
the ESBG to the other fiber when the ESBG has a selected index.
68. A component as claimed in claim 67 wherein the fibers have
substantially the same indexes, and wherein when said ESBG is in
its first state substantially all light applied to one of said
fibers is transferred through the ESBG to the other fiber.
69. A component as claimed in claim 68 wherein said ESBG has a
selected grating period, said ESBG, when in a second state as a
result of the voltage applied theracross, blocking light of at
least one selected wavelength from a multiwavlength optical signal,
determined by the period of the ESBG grating, from passing through
the ESBG, light at such at least one wavelength continuing to
propagate in its original path.
70. A component as claimed in claim 67 wherein the fibers have
different indexes, wherein said ESBG has a selected grating period
.LAMBDA., wherein the propagation constants for the two fibers are
.beta..sub.i and .beta.'.sub.i respectively, and wherein for at
least one wavelength of a multiwavelength light signal, the
condition 2.pi./.LAMBDA.=.beta..sub.i-.- beta.'.sub.i is satisfied,
there being no coupling between fibers through the ESBG except for
the at least one wavelength for which said condition is
satisfied.
71. A component as claimed in claim 67 wherein the fibers have
different indexes, wherein said ESBG has a gating with an index
contrast .DELTA.n, and wherein the difference in the effective
index of the fibers is .gtoreq..DELTA.n.
72. An ESBG characterized in that the ESBG has a subwavelength
grating.
73. An ESBG as claimed in claim 72 wherein said grating has a
period substantially less than 0.5 .mu.m.
74. A method of fabricating an ESBG having a subwavelength grating
of period .LAMBDA. including exposing a H-PDLC film by one of (a)
exposing the film with two interfering light beams of suitable
wavelength, the half angle .theta. between the beams being large
enough so that sin .theta.=.lambda./2.LAMBDA., where .lambda. is
the center wavelength of a light signal with which the ESBG is to
be utilized; (b) exposing the film through a suitable binary phase
mask; (c) exposing the film through a master grating.
75. A method of fabricating an integrated optical network having a
plurality of nodes, with at least one ESBG formed at each of said
nodes, including: forming selected optical waveguides in a
substrate, at least selected ones of said waveguides passing
through selected ones of said nodes; forming an ESBG with
electrodes at each node; and covering the waveguides and ESBGs.
76. A method as claimed in claim 75 wherein said forming an ESBG
step includes forming an electrode film and a H-PDLC film at each
node, and exposing the H-PDLC film at each note to form and ESBG
grating thereon.
77. A method as claimed in claim 76 wherein said covering step
includes forming a second electrode film on a cover plate at each
node; and covering the waveguides/H-PDLC films with said cover
plate, each electrode on the coverplate overlying the H-PDLC film
for the corresponding node.
78. A method as claimed in claim 76 wherein said exposing step
includes one of (a) exposing each H-PDLC film with two interfering
light beams of suitable wavelength, the beams being at a selected
angle to each other; (b) exposing all of the films simultaneously
through a suitable binary phase mask; and (c) exposing each of the
films through a suitable mask.
79. A method as claimed in claim 76 wherein said forming waveguides
step includes forming selected optical waveguides in a first
substrate and in a second substrate, which substrates are mounted
adjacent each other during said covering step, the waveguides on
the two substrates intersecting at at least selected nodes.
80. A method as claimed in claim 79 wherein a H-PDLC film with ESBG
gratings formed therein at said nodes is one of (a) formed on one
of said substrates, and (b) independently formed and mounted
between said substrates.
81. A method as claimed in claim 79 said forming electrode film
step includes forming an electrode on each substrate on a waveguide
at each node.
82. An integrated optical network having N guided wave optical
inputs, and M guidewave optical outputs, said network including: an
optical waveguide connected to each input and to each output, said
waveguides intersecting at nodes; and an ESBG switch component at
each node which is operative to either pass an optical signal on
one waveguide intersecting at the node on the one waveguide or to
transfer at least a portion of such optical signal to the other
waveguide intersecting the node, depending on the state of the
ESBG.
83. A network as claimed in claim 82 wherein optical signals on
said guided wave optical inputs are multiwavelength WDM signals,
and including an ESBG switch component at each node for each
wavelength to be transferred at the node.
84. A network as claimed in claim 82 wherein all of said waveguides
have substantially the same index n.sub.1, and wherein the ESBG
switch component at each node includes an ESBG having a grating
with an index contrast .DELTA.n in optical contact with each
waveguide, and at least one waveguide interconnecting the ESBGs,
said waveguide having an index n.sub.2, where
n.sub.1-n.sub.2.gtoreq..DELTA.n.
Description
PRIOR APPLICATIONS
[0001] This application is a continuation in part of application
Ser. No. 08/797,950 filed Feb. 12, 1997 (the '950 application) and
claims priority from provisional specification 60/055,571 filed
Aug. 13, 1997, the subject matter of both the parent application
and the provisional being incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates to switchable optical components,
particularly ones utilizing guided wave optics and to ones
particularly adapted for use in wavelength division mutliplexing
(WDM) systems, including switchable waveguide gratings,
particularly ones used for switchable add/drop filtering (SADF) and
wavelength selective cross-connect (WSXC), to designs utilizing
coupler-halves or side-polished fibers, to attenuators and other
optical grating components utilizing very small period optical
grating elements, to other wavelength selective or wavelength
independent components, to devices using such components, to
methods for the fabrication of such components and devices, and to
integrated structures for such devices.
BACKGROUND OF THE INVENTION
[0003] Because of their very large bandwidth capacity, optical
signals are being increasingly utilized for the transmission of
data. Further, the bandwidth capacity of a given fiber optic cable
can be further increased by transporting a multiplicity of
independent signals within a single fiber on separate channels at
slightly different wavelengths, a technique known as wavelength
division multiplexing (WDM). Thus, for example, a nominally 1550 nm
fiber optics signal might comprise four, eight, 64, 80 or more
channels, each separated by for, example approximately 0.8 nm
(corresponding to 100 GHz) or approximately 1.6 nm (corresponding
to 200 GHz). Multiwavelength operation facilitates an increasingly
important advantage of optical transport and switching which is
that several non-interacting signals may pass through the switch
simultaneously, which signals convey entirely incompatible data
rates, encodings and protocols in parallel without compromising one
another. However, for such signals to be useful, it must be
possible to wavelength selectively switch the optical signals
coming in on an optical fiber, bus (or other optical conduit) to a
fiber/conduit leading to a desired drop/destination, to wavelength
selectively add signals from a drop to the bus or to wavelength
selectively transfer signals between fibers or other optical
conduits. The first two functions are sometimes referred to as
switchable add/drop filtering (SADF) and the last function is
sometimes called wavelength selective cross-connect (WSXC). In
other applications, switching the entire fiber signal, inclusive of
all wavelength channels, is required (such switching sometimes
being denoted as "space switching"). In complex fiber optic
structures such as those used in the telecommunications industry
and for sensor and computer data networks, light signals must be
efficiently routed or switched from an array of N incoming optical
fibers, which fibers may be single mode or multimode, to an array
of M outgoing optical fibers. Such a space switch will sometime be
referred to hereinafter as an N.times.M switch or
cross-connect.
[0004] While a number of techniques have been proposed over the
years for performing N.times.M switching optically, none of these
techniques have proved to meet all requirements simultaneously.
This is partly due to the varied architectures which are required
for such switches. For example, an N fiber in, N Fiber out
(N.times.N) switch that maps each incoming fiber optical signal to
one and only one fiber output is termed an N.times.N cross-connect.
It is nonblocking if any connection is possible, without regard to
earlier established connections. For some applications,
reconfigurably nonblocking switches are sufficient. In other
applications, switches that multicast or broadcast, sending one
incoming signal to more than one output, or that perform other
variant functions, are required. The data capacity demands on fiber
optic networks are also becoming more complex, imposing a
requirement that switching technologies be scalable so as to be
extendable in a straight forward manner from small switches (for
example 2.times.2 or 4.times.4 to larger switches such as
64.times.64, 1024.times.1024, and beyond). It is also desirable
that such switches be integrable such that individual miniaturized
switching elements can be combined with many others on a single
chip or substrate to provide a larger N.times.N or N.times.M
cross-connect structure. However, designing such structures,
particularly for larger switches, is very complex even for single
channel operation, and the complexity increases dramatically for
multichannel WDM operation (i.e., wavelength selective switching
with an N.times.M.times.m switch, where m is the number of WDM
channels).
[0005] Another requirement for optical switches of the type
described above in particular, and for optical components and
structures in general, is that they efficiently interface with
optical fibers, the use of which to transport high bandwidth
signals over long distances is increasingly prevalent, in a manner
so as to minimize coupling losses. Other key performance parameters
include minimizing insertion loss, crosstalk and polarization
sensitivity, insuring good optical isolation in all switch states,
good spectral bandwidth, and good dynamic range for on/off contrast
ratio. Low operating power, high switching speed, low power
consumption, stability, long service life/temperature insensitivity
and high reliability are also important. However, for many network
reconfiguration and protection switching functions, switching
speeds in the range of 1 microsecond to 1 millisecond are adequate
and sufficient.
[0006] Further, in the present state of the art, neither space
switching, nor wavelength selective switching techniques, are
entirely satisfactory. One reason for this is that the various
network control and reconfiguration functions required have
generally been met by different and incompatible technologies.
Optical network systems would be considerably advanced, in
efficiency, manufacturability and cost, if several disparate
network control functions could be implemented on the basis of a
single underlying technology.
[0007] All-optical switching is increasingly regarded as essential
for future networks. Because satisfactory products for performing
such optical switching have not existed, it has therefor been
necessary to convert optical signals to be switched into electrical
signals for switching and to then reconvert the signals to optical
signals for outputting. This technique can be expensive, time
consuming, impose bandwidth limitations on the system and introduce
several sources of potential error. It can also limit the
flexibility of the system and is generally not an efficient way to
operate.
[0008] In addition to the switching applications discussed above,
there are numerous applications where a need exists to be able to
change the direction in which an optical signal is passing through
a waveguide, dynamically filter an optical signal, particularly a
multiwavelength or multichannel signal, so as to selectively add,
drop, pass or block various of the individual wavelengths or
channels (or the entire signal), to selectively attenuate an
optical signal, including one or more signals of a multiwavelength
or multichannel line, to selectively crossconnect optical paths
including multichannel or multiwavelength optical paths to
facilitate the transfer of one or more channels therebetween,
and/or to selectively couple the multiwavelengths or multichannels
along optical paths out of the plane of the waveguide. It should be
possible to perform all of these functions utilizing optical
components and/or structures which are relatively easy and
inexpensive to fabricate. In particular, it would be desirable if
fabrication techniques could be provided which would permit complex
optical networks to be fabricated utilizing a parallel,
simultaneous, one-shot fabrication techniques that incorporates a
multiplicity of functionalities on a single chip for the
implementation of space switching, wavelength selective switching,
switchable add-drop filtering, wavelength selective cross-connect
switching, together with such additional functions as programmable
attenuation, all on a single chip and using a single material
technology rather than requiring each component to be separately
fabricated.
Definitions
[0009] In the following sections, various terms will be used, which
terms should be considered to have the following definitions:
[0010] "Bragg gratings or gratings" are periodic structures formed
by spatially varying refractive index distributions or similar
perturbations throughout a defined volume or the boundary of a
guiding region. Simple Bragg gratings are periodic in one
dimension. More complex diffractive structures, which for purposes
of this invention will also be encompassed within this definition,
may be volume holograms, diffractive lenses, or other computer
generated or optically recorded diffractive index distributions, in
most cases permeating a substantially three-dimensional volume,
designed and fabricated for purposes of coupling an incident laser
or other light beam or a light beam received through guided wave
optics into a desired output state or mode either one guided mode
to another guided mode, a guided mode to a free space mode or vice
versa.
[0011] "Switchable gratings or switchable Bragg gratings" are
volumetric gratings whose grating period index variation can be
modulated, induced or caused to vanish by application of an
electric field. These differ from standard gratings which are not
switchable. This definition does not imply altering the period of
the grating, but only the amplitude of the spatially varying index
variation. A switchable grating, in the simplest example, may be
described as a grating element, that, in its switched-on state, has
filtering or other diffractive properties comparable to a high
quality conventional fiber or waveguide Bragg grating, of either
the transmission or reflection type, and in its switched-off state,
effectively vanishes to be replaced with a low loss waveguide or
volume of transparent optical material.
[0012] Until recently, few mechanism were available for switchable
Bragg gratings. Certain semiconductor gratings can be switched, but
only in limited geometrical configurations, and the dynamic range
for control of the spatial index modulation is relatively small.
Liquid crystal gratings, usually formed by physically structured
electrodes, may be switchable, but are primarily relevant to free
space non-volumetric gratings, are excessively scattering for use
with fiber optic signals and are relatively slow, switching being
in the millisecond range. All manner of switchable gratings that
involve the use of structured electrodes to produce the spatial
periodicity, such as magneto-optic materials and lithium niobate
materials, are limited in their application in that the spatial
period and depth of grating are dependent on the lithographic
processes of fabricating electrode patterns. Such structured
electrode gratings are not practical at spatial periodicities much
less than one micrometer, which smaller periods are essential in
various fiber optic applications, and also do not tend to produce
volumetric (Bragg) gratings since the electrode periods cannot
penetrate unlimited volumes.
[0013] "Holographic polymer/dispersed liquid crystals or H-PDLC"
are any microdroplet composite of liquid crystal (LC) in polymers
or other morphological variants including polymer networks with
interpenetrating LC. This family of switchable gratings is based on
microdroplet dispersions of liquid crystals in a polymer host, the
volume gratings containing periodic structures with periods as
small as 200 nanometers or less which are achieved by holographic
recording and photopolymerization processes. The switching of such
gratings is achieved by applying an electrical field by means of a
uniform, monolithic electrode to the entire grating region, as
opposed to producing the grating by patterning the electrodes.
Examples of H-PDLC include, but are in no way restricted to, the
following:
[0014] 1. R. L. Sutherland, L. V. Natarajan, V. P. Tondiglia, T. J.
Bunning, Bragg Gratings in an Acrylate Polymer Consisting of
Periodic Polymer-Dispersed Liquid-Crystal Planes, Chem. of
Materials, 1993, 5, 1533.38.
[0015] 2. R. L. Sutherland, L. V. Natarajan, V. P. Tondiglia, T. J.
Bunning and W. W. Adams, Development of Photopolymer/Liquid Crystal
composite Materials for Dynamic Hologram Applications, Proc. SPIE
Vol. 2152, paper 38.
[0016] 3. V. P. Tondiglia, L. V. Natarajan, R. L. Sutherland, T. J.
Burning and W. W. Adams, Volumn holographic image storage and
electro-optic readout in a polymer dispersed liquid crystal film,
Opt. Lett. v. 20, p. 1325, 1995.
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Bunning and W. W. Adams, Switchable holograms in a new
photopolymer-liquid crystal composite, Proc. SPIE, Vol. 2404, p.
132, 1995.
[0018] 5. R. L. Sutherland, L. V. Natarajan, V. P. Tondiglia, T. J.
Bunning and W. W. Adams, Electrically switchable volumn gratings in
PDLC, Appl. Phys. Lett., Vol. 64, p. 1074, 1994.
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et al.
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et al.
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Peyghambarian, Photorefractive Polymer-Dispersed Liquid Crystals,
Optics Letters, Vol. 22, No. 16, p. 1226-1228, Aug. 15, 1997.
[0022] 9. Keiji Tanaka, Kinya Kato, Shinjui Tsuru, and Shigenobu
Sakai, Holographically Formed Liquid-Crystal/polymer Device for
Reflective Color Display, Journal of the SID, Vol 2, No. 1, p.
37-40, 1994.
[0023] 10. Emily W. Nelson, Adrian D. Williams, Gregory P.
Crawford, Louis D. Silverstein, and Thomas G. Fiske, Full-Color
Reflective Displays, IS&T's 50th Annual Conference, p.
669-673.
[0024] 11. G. Crawford and S. Zumer, Liquid Crystals in Complex
Geometries Formed by Polymer and Porous Networks, Taylor and
Francis, 1996, London.
[0025] "Electronically switchable Bragg grating or (ESBG)" any of
an extensive range of devices and device geometries realized
utilizing the H-PDLC materials technology.
SUMMARY OF THE INVENTION
[0026] In accordance with the above, this invention provides a
component for selectively reconfiguring an optical signal on a
guided wave optical path which path may for example be a waveguide
formed in a composite optical structure. The component includes at
least one ESBG in optical contact with the path, and electrodes for
selectively applying at least a first and a second voltage across
the ESBG, there being no change in the optical signal for the path
when the first voltages is across the ESBG and there being a
selected change in the optical signal on the path when the second
voltage is across the ESBG. Where the optical signal is a
multichannel WDM signal, a selected channel may be dropped from the
path when the second voltage is across the ESBG, the dropping
occurring either by the channel being reflected back through the
path or being dropped/outcoupled from the path through the
ESBG.
[0027] The component may include a second guided wave optical path
which is optically coupled to the ESBG, with at least one selected
channel being transferred between the paths when the second voltage
is across the ESBG. In particular, a channel may be either dropped
or added to the optical path by being transferred between the
optical path and the second optical path through the ESBG when the
ESBG has a second voltage thereacross. Where WDM signals appear on
both paths, WSXC transfer of at least a selected channel may be
performed between the paths when a second voltage is across the
ESBG.
[0028] A plurality of ESBGs or ESBG components may be optically
coupled to both paths, with a different channel being transferred
between the paths by each ESBG when the second voltage is
thereacross. For some embodiment of the invention, there is an ESBG
in optical contact with each of the paths and at least one optical
path interconnecting the ESBGs, there being two optical paths
interconnecting the ESBGs to form a ring for a generally preferred
embodiment. One of the rings may be provided between the paths for
each WDM channel to be transferred between the paths. The two
signal-carrying optical paths preferably have the same first
effective index, with the optical paths of the rings having a
second effective index different from that of the first index, the
different effective indices being achieved for example by having
the optical paths of the ring of different size than that of the
main optical paths.
[0029] The optical paths may also have different indices which may
be accomplished for example by having the optical paths be of
different size. The difference in the indices of the gratings is
preferably greater than or equal to the index contrast .DELTA.n of
the ESBG grating.
[0030] Rather than being integrated optics, the component may also
be a switchable coupler half device, for example a switchable drop
filter, a switchable outcoupler, or a switchable attenuator. For
such an attenuator, the ESBG has a submicron grating.
[0031] The ESBG may also be part of a resonator. When the resonator
is being used in a channel drop filter, the component includes a
first drop resonator and second reflector resonator spaced from
each other in the direction of travel of the optical signal on the
optical path by an integer number of wavelengths of a wavelength to
be dropped, plus a half wavelength. Each resonator may also be a
multipole resonator formed of resonator sections which are either
series coupled or parallel coupled. The resonator component may
also be a guide channel dropping filter with the resonator being
between the optical paths. The resonator may also be a split
resonator having a phase delay section between the split resonator
sections.
[0032] Where there are two waveguides or optical paths, the ESBG
may be in the cladding for both optical paths which claddings
overlap. Where the claddings of the two optical paths do not
overlap, the ESBG may extend over or overlie both optical paths to
affect interconnection. For any component involving two optical
paths, sidelobes may be suppressed by apodization. As previously
indicated, all of the above are preferably effected through use of
integrated optics technology except for the half coupler
embodiments.
[0033] For half coupler embodiments, an optical fiber having a core
with cladding therearound is provided, the core having an index
n.sub.1, the cladding having an index n.sub.2, and the effective
index of the fiber being n.sub.e, the cladding being at least
partially removed in a selected region. An ESBG is mounted to the
fiber in the region, the ESBG having an index n.sub.B when in a
first state, where n.sub.B is substantially equal to n.sub.2.
Electrical elements including electrodes are also provided for
selectively applying a voltage across the ESBG, the effect which
the ESBG has on light applied to the ESBG varying as the voltage
thereacross changes, thereby changing the state of the ESBG. Light
applied to the optical fiber is substantially unaffected by the
ESBG when the ESBG is in its first state, this for example
occurring when the mechanism applies substantially no voltage
across the ESBG. Variation in n.sub.B as a result of voltage
changes across the ESBG may result in selective attenuation of
light applied to the fiber, particularly if the ESBG has a grating
with a subwavelength. Such attenuation is generally substantially
independent of wavelength.
[0034] The ESBG may cause light at one or more selective
wavelengths to be coupled through the ESBG in at least one
direction to or from the fiber as the voltage across the ESBG is
varied from the voltage causing the ESBG to be in its first state.
The ESBG grating may also have a period which causes light at one
or more selected wavelengths to be reflected back along the fiber,
thereby filtering such one or more selected wavelengths from light
propagating in the fiber. Two of the fibers may be provided, with
the regions of the fibers having cladding remove being adjacent and
with an ESBG being mounted between the fibers in both regions. For
this configuration, all light on one of the fibers passes unchanged
through the ESBG to the other fiber when the ESBG has a selected
index. When the fibers have substantially the same index, a light
applied to one of the fibers is transferred through the ESBG to the
other fiber when the ESBG is in its first state. When the ESBG is
in a second state as a result of a voltage applied thereacross,
light of at least one selected wavelength determined by the period
of the grating is blocked from passing through the ESBG, such light
continuing to propagate on the original fiber. The fibers may also
have different indices in which case, for at least one wavelength
of a multiwave light signal for which the condition
2.pi./.LAMBDA.=.beta..sub.i-.beta.'.sub.i is satisfied, where
.LAMBDA. is the period of the grating and .beta..sub.i,
.beta.'.sub.i are the propagation constants for the two fibers
respectively, there is coupling between the fibers only for such
wavelength. Where the fibers have different indices, it is
preferable that the difference in their effective index be greater
than or equal to the index contrast .DELTA.n of the ESBG
grating.
[0035] The invention also includes providing an ESBG having a
subwavelength grating, which grating may have a period
substantially less than 0.5 .mu.m. Such a subwavelength grating may
be obtained by exposing a H-PDLC film by one of (a) exposing the
film with two interfering light beams of suitable wavelength, the
half angle .theta. between the beams being large enough so that sin
.theta.=.lambda./2.LAMBDA., where .lambda. is the center wavelength
of a light signal with which the ESBG is to be utilized; (b)
exposing the film through a suitable binary phase mask; (c)
exposing the film through a master grating.
[0036] Integrated optical networks each having a plurality of
nodes, with at least one ESBG formed at each node, may be formed by
forming selected optical waveguides in a suitable substrate, at
least selected ones of the waveguides passing through selected ones
of the nodes, forming an ESBG with electrodes at each node and
covering the waveguides and ESBGs. The ESBGs may be formed by
forming an electrode film and a H-PDLC film at each node, and
exposing the H-PDLC film at each node to form the ESBG grating
thereon. The covering of the waveguides and ESBGs may be
accomplished by forming a second electrode film on a cover plate at
each node and covering the waveguides/H-PDLC film with the
coverplate, each electrode on the coverplate overlying the H-PDLC
film for the corresponding node. The exposing of the H-PDLC film
may be accomplished by one of (a) exposing each H-PDLC film with
two interfering light beams of suitable wavelengths, the beams
being at a selected angle to each other; (b) exposing all the films
simultaneously through a suitable binary phase mask; (c) and
exposing each of the films through a suitable mask. Selected
optical waveguides may also be formed in a first substrate and in a
second substrate, which substrates are mounted adjacent each other
during the covering step, the waveguides on the two substrates
intersecting at at least selected nodes. For this embodiment, a
H-PDLC film, with ESBG gratings formed therein at the nodes, is
either formed on one of the substrates or independently formed and
mounted between the substrates. An electrode film may be formed on
each substrate on a waveguide at each node.
[0037] Finally, the invention includes an integrated optical
network having N guided wave optical inputs and M guided wave
optical outputs, the network including an optical waveguide
connected to each input and to each output, the waveguides
intersecting at nodes, and an ESBG switch component at each node
which is operative to either pass an optical signal on one
waveguide intersecting at the node on the waveguide or to transfer
at least a portion of such optical signal to the other waveguide
intersecting at the node, depending on the state of the ESBG. For
WDM signals, an ESBG switch component may be provided at each node
for each wavelength to be transferred at the node. For preferred
embodiments, all the waveguides have substantially the same index
n.sub.1, the ESBG switch component at each node includes an ESBG
having a grating with an index .DELTA.n in optical contact with
each waveguide and at least one waveguide interconnecting the
ESBGs, the waveguide connecting the ESBGs having an index n.sub.2,
where n.sub.1-n.sub.2.gtoreq..DELTA.n.
[0038] The foregoing and other objects, features and advantages of
the inventions will be apparent from the following more particular
description of preferred embodiments of the inventions as
illustrated in the accompanying drawings.
IN THE DRAWINGS
[0039] FIGS. 1a and 1b are schematic representations of a waveguide
with an ESBG in the core region and cladding region
respectively.
[0040] FIGS. 2a and 2b are schematic representations of a
transmission ESBG in the off-state and on-state respectively.
[0041] FIGS. 3a and 3b are schematic representations of a
reflection ESBG in the off-state and on-state respectively.
[0042] FIGS. 4a and 4b are schematic representation of a single
switchable waveguide component in the on-state and off-state
respectively.
[0043] FIG. 5a is a schematic representation of a switchable
add/drop single channel filter in accordance with the teachings of
this invention.
[0044] FIG. 5b is a schematic representation of the various
coupling modes which potentially exist in the filter of FIG.
5a.
[0045] FIG. 5c are graphs illustrating optical performance at
various ports for the filter of FIG. 5a without apodization and
FIG. 5d are graphs of the performance at these port with
apodization.
[0046] FIG. 5e is a schematic representation of an integrated
planar four-channel array filter utilizing the components of FIG.
5a.
[0047] FIG. 5f is a schematic representation of an integrated
optics crossconnect switch.
[0048] FIGS. 6a and 6b are schematic representations illustrating
two embodiments for crosspoint switches in accordance with the
teachings of this invention.
[0049] FIG. 7a is a graph comparing the filter response of the
crossconnect shown in FIG. 6a with the filter of FIG. 5a.
[0050] FIG. 7b is a graph showing details of the throughput
response for the crossconnect of FIG. 6a.
[0051] FIGS. 7c and 7d are graphs illustrating typical optical
responses for the crossconnect shown in FIG. 6b.
[0052] FIGS. 8a and 8b are schematic representations of
illustrative crossconnect arrays between multiple waveguides for
multiple wavelengths utilizing, the crossconnect elements of FIG.
6b.
[0053] FIG. 9 is a schematic representation of an ESBG being used
as part of a resonator.
[0054] FIG. 10a is a schematic representation of a resonator
embodiment for a channel drop filter.
[0055] FIGS. 10b and 10c are examples of such filter employing
higher order coupled resonators.
[0056] FIGS. 10d-10f are schematic representations for three
guide-channel dropping filters employing resonators in accordance
with the teachings of this invention.
[0057] FIG. 11 is a graph of an illustrative response for a matched
bus resonator of the type shown in FIG. 10a.
[0058] FIGS. 12a-12e illustrate a method for the fabrication of an
integrated array in accordance with the teachings of this
invention.
[0059] FIGS. 13a and 13b illustrate an alternative fabrication
technique for integrated arrays in accordance with the teachings of
this invention.
[0060] FIG. 14a is a schematic representation of a coupler half
device in accordance with the teachings of this invention and FIG.
14b is a chart of indices and dimensions for various components of
for device of the type shown in FIG. 14a.
[0061] FIGS. 14c and 14d are sectional views along the line c-c in
FIG. 14a with the electrode on top and bottom of the ESBG and on
the sides of the ESBG respectively.
[0062] FIG. 15 is a schematic representation of a switchable Bragg
filter employing a coupler half device in accordance with the
teachings of this invention.
[0063] FIG. 16 is a schematic representation of a method for
fabricating a device of the type shown in FIG. 15.
[0064] FIG. 17 is a schematic representation of a switchable
outcoupler utilizing coupler half technology in accordance with the
teachings of this invention.
[0065] FIG. 18 is a schematic representation of a tunable
attenuator utilizing coupler half technology in accordance with the
teachings of this invention.
[0066] FIG. 19a is a graph showing the optical power transmitted
through the fiber as a function of electro-optically altered ESBG
index for two different polarizations and FIG. 19b is a chart
illustrating the power transmitted as a function of wavelength for
three levels of attenuation.
[0067] FIG. 20 is a schematic representation of a channel add/drop
crossconnect device utilizing half coupler technology in accordance
with the teachings of this invention.
[0068] FIG. 21 is a schematic representation of a 2.times.2 space
switch utilizing half coupler and ESBG technology.
[0069] FIGS. 22a and 22b are schematic representations of a grating
assisted coupler between two fibers.
[0070] FIG. 22c is a graph illustrating performance for the coupler
of FIGS. 22a, 22b.
DETAILED DESCRIPTION
[0071] The '950 application teaches a variety of free space,
waveguide and fiber optic components utilizing ESBG technology to
perform various switching, reflection, filtering, routing and other
functions for optical signals on single wavelength or
multiwavelength lines. Bragg gratings may be classified as
freespace or waveguide gratings, and within each category as
transmission or reflection gratings. ESBGs corresponding to each
such type converting each to a switchable form. As illustrated in
FIG. 1A, an ESBG 12 may be located in the core or guiding region 14
of a planar waveguide or, as illustrated in FIG. 1B, the ESBG 12
may be in the evanescent or cladding region 16 thereof. As was
indicated in the '950 application, ESBGs may be utilized to perform
a variety of functions; however, these functions may be generally
characterized as transmission or reflection functions. Since the
desirable characteristics and appropriate designs of ESBGs when
used for transmission are different than when used for reflection,
the ESBGs employed for these functions are also somewhat different
in their geometry, materials properties, periods, magnitude of
spatial index modulations, and other parameters.
[0072] In particular, transmission gratings are used for the
spatial diversion of beams into an alternative path, primarily
without discrimination as to wavelength channel, and are therefore
useful for 2.times.2 space switches, including free space, optical
fiber and planar waveguide 2.times.2 space switches. (The term
"space switches" should not be confused with "free space switches."
Space switching means diversion of the path of an entire beam or
guided wave. Free space propagation refers to unguided light
beams.) Since this application relates mostly to guided waves in
waveguides and/or optical fibers rather than free space switching,
free space 2.times.2 switches will not be discussed further, the
discussion below relating almost entirely to guided wave
structures.
[0073] Transmission Gratings
[0074] For transmission gratings, the diffraction efficiency at the
Bragg matched angle and wavelength is given by the approximate
formula
d.e.=sin.sup.2.pi.[.DELTA.nL/.lambda. cos .theta.] Eq. (1)
[0075] where .theta. is the Bragg angle, .lambda. is the
wavelength, L is the interaction length, and .DELTA.n is the
magnitude of the spatial index modulation, which can be controlled
over a wide range, (approximately .DELTA.n=0.001-0.05) by varying
the formulation and processing of the H-PDLC, this being one of the
advantages of this material system for ESBGs.
[0076] Complete diffraction means that d.e.=1, or .DELTA.n
L/.lambda. cos .theta.=1/2. (Although the principle remains valid,
this formula varies significantly when describing coupling within a
single mode waveguide, and should be further modified depending on
whether the grating is in the core or overlay region, but as a
general rule it shows the essential dependencies on basic
parameters.) For beam steering efficiency, it is desirable for
.DELTA.n in a transmission grating to be relatively large, for
example 2-5%, and thereby for L to be small, for example
.apprxeq.50 .mu.m. In such a grating, the wavelength dependence of
the coupling is relatively weak, since a 1% change in .lambda.
would cause only about a 1% change in coupling strength.
[0077] The orientation of an ESBG 12 having a transmission grating
and its electrodes 20 are indicated in FIGS. 2a and 2b; the Bragg
planes are as indicated, with the grating vector 22 approximately
transverse to the direction of the incident beams 24. The grating
vector is a term of art for a vector perpendicular to the planes of
the grating, whose length or magnitude is inversely proportional to
the spacing of the planes. In FIGS. 2a and 2b, the grating vector
is in the "y" direction. While FIGS. 2a and 2b illustrate in the
simplest case a free space transmission gratings, it should also be
understood that similar structures may be placed in guided wave,
confined regions.
[0078] Looking further at FIGS. 2a and 2b and the ellipsoidal
"footballs" 26 of liquid crystal microdroplets illustrated as being
distributed within the Bragg planes of the ESBG film, it can be
seen that in order to effectively reorient the liquid crystal
molecules within each droplet, and to apply the voltage across the
shortest possible distance to produce the largest possible field,
then in the free-space case, only one choice for for electrode
positions is available with respect to the substantially planar
structure shown. Viewing the devices from above the planar
structure, the electrodes must be on the "front and back" of the
grating (considering the side where light enters to be the front).
However, for transmission gratings in a waveguide geometry, where
the three dimensions of the grating volume may be approximately
comparable, additional choices are available, and the electrodes
may be applied as described above or else to the top and bottom (or
right side/left side). Although either of these geometries can be
considered useful in principle, in the case of waveguide gratings,
the substantially planar structure and thinness of the films
dictate that the top-bottom electrode placement is normally the
practical choice. (For nonplanar, free space applications in which
the ESBG is not a thin film substantially parallel to the
propagation of the light, but rather a thin film normal to the
propagation of the light, either choice would be viable, with the
choice in specific applications depending on the birefringence of
the liquid crystals and their index relationships to the other
components.)
[0079] The physical locus of a transmission grating containing ESBG
within the planar device may be in either the core region 14 or the
overlay (also known as the cladding or evanescent) region 16, as
shown in FIGS. 1a and 1b. The interaction with the grating is
stronger in the former case, but losses may be substantially
reduced in the latter case. A choice between these alternative
designs may be dictated for specific devices based on numerical
modeling and computer simulations which predict performance to a
high degree of accuracy, including estimates of losses, and allow
evaluation of relative advantages of alternative fabrication paths.
In general however, it may be considered that both geometries are
available for use with H-PDLC based ESBGs and for many purposes may
be functionally equivalent. Notwithstanding this functional
equivalence, the overlay geometry provides relative ease of
fabrication, together with advantages of using continuous, low-loss
waveguides or optical fibers; however, for many applications the
core geometry provides superior overall performance.
[0080] Reflection Gratings
[0081] ESBGs 12 containing reflection gratings in a planar,
waveguide context are produced so that the orientation of the Bragg
planes is as indicated in FIGS. 3a and 3b, thereby reflecting part
of the propagating wave into the backward direction, with the
center of wavelength band reflected determined according to the
formula .lambda.=2n.sub.eff.LAMBDA.- , where n.sub.eff is the
effective mode index of the film and .LAMBDA. is the physical
period of the grating. In reflection gratings, it is desirable to
emphasize the wavelength selective properties because the primary
application is switchable wavelength filtering for WDM channel
discrimination. The spectral width of the reflection notch must be
calculated using known mathematical models of waveguide design, but
is approximately 1 3 4 [ ( L ) ( n n ) ] 1 / 2 Eq . ( 2 )
[0082] In this case, unlike the transmission grating, it is desired
that .DELTA.n/n be as generally small, 0.1% or less, and L should
be as long as possible, up to 2000-3000 .mu.m. In the art of H-PDLC
chemistry, this can be accomplished by adjusting downward the
amount of liquid crystal contained in the pre-ESBG solution or
otherwise altering the chemical formulation and varying processing
conditions accordingly. The reflectivity of a reflection grating
exactly at the center wavelength is approximately
Reflectivity=1-4e.sup.[-(.DELTA.n/n)(L/.LAMBDA.)] Eq. (3)
[0083] which is desired to be nearly=1. This can be accomplished
even if .DELTA.n/n is small (about 0.001) provided L/.LAMBDA. is
large (about 4000).
[0084] Thus whereas transmission gratings for space switching (beam
steering or coplanar coupling) are optimized when L is small and
.DELTA.n is large (which for educative purposes may be informally
described as a "short, fat grating), "reflection gratings for
wavelength filtering are optimized when L is long and .DELTA.n is
small (long, skinny grating). A major advantage of the H-PDLC
materials family underlying ESBGs is the ability to accommodate
both of these requirements within one family of devices, and even
applying this variation to several elements on one device
substrate, by altering formulation and processing over a range not
possible with other materials family, and therefore to be able to
combine these two disparate classes of elements within one
manufacturing process, on one substrate. This range of possibility
does not exist, for example, with semiconductor gratings
illustrating the multifunctionality disclosed herein with respect
to ESBGs.
[0085] A further distinction between transmission and reflection
gratings may be seen in FIGS. 2a-3b in terms of electrode
placement. The grating vector 22 in a reflection grating is located
substantially collinearly with the forward and backward propagating
light (i.e. in the "x" direction), a completely different direction
than for transmission gratings where the grating vector is in the
"y" direction. The aspherical microdroplets 26 ("footballs of
liquid crystals") within the ESBG 12 are always oriented parallel
to the grating vector; thus the microdroplets for reflection
gratings are elongated in the same direction as the light
propagation (unlike the transmission grating, where the elongation
was substantially perpendicular to the direction of light
propagation). Consideration of the liquid crystal "footballs" and
the orientation of the LC molecules within them shows that in order
for the electric field to be effective in reorienting them, the
electrodes for a reflection grating can only be placed on the top
and bottom of the grating or on the sides of the grating. To
optimize the effect of the electric signal, the electrodes should
be across the shorter one of these dimensions. This is
straightforward in the case of thin film reflection gratings. All
of the planar devices to be hereinafter discussed, both
transmission gratings and reflection gratings, will use electrodes
20 parallel to the plane of the substrate and coated in layers
above and below the ESBG overlay film, since this is normally the
preferred configuration. However, other configurations are within
the contemplation of the invention.
[0086] As in the case of transmission gratings, reflection gratings
can in principle be placed either in the core region 14 or overlay
region 16 with similar functionality, and the
advantages/disadvantages of each will also be the same as for
reflection gratings.
[0087] Since the detailed physics and chemistry of H-PDLC may be
understood from the cited references, the simplified physical model
displayed in FIGS. 2 and 3 is solely for the purpose of assisting
with a physical appreciation of the fundamentals of ESBG design
variants using H-PDLC. Therefore, the description herein of H-PDLC
microdynamics may be oversimplified or may be altered by improved
scientific understanding of H-PDLC in the future, without
significantly altering or limiting the inventions herein
described.
[0088] Requirements on a H-PDLC Formulation for Waveguides.
[0089] There are two requirements on the refractive index n of the
H-PDLC formulation that will lead to low loss structures with the
functional flexibility described above.
[0090] First, the H-PDLC formulation must be subject to chemical
adjustment of proportions such that the index spatial modulation
.DELTA.n/n can be designed over a wide range after holographic
exposure, from less than 0.01% (reflection gratings, long and
skinny) up to 5% (transmission gratings, short and fat). Whatever
index spatial variation is produced in this way in the unpowered
state of the grating, this spatial variation will subsequently be
controlled, increased or decreased in amplitude, as an electric
field is applied to the film on the order of for example 1-10
V/.mu.m. Whether the electrical field acts on the one hand to
suppress the spatial index modulation, ideally to point of
vanishing, or alternatively if the electrical field acts to induce
a substantial index modulation, depends on the details of liquid
crystal droplet morphology, whether the gating is used in a
transmission or reflection mode, the direction and polarization of
the light guided wave, and other factors. The inventions
contemplated here may therefore be switched between grating-active
and grating-inactive states either by the application or removal of
an appropriate electrical field, which difference does not
substantially alter the described optical functionality.
[0091] Also, it is highly desirable in certain of these structures
to provide a H-PDLC formulation that results, after exposure and
processing, in an average index n that varies (over the same range
resulting from application of an electric field) over the typical
single mode dielectric core/cladding range of 1.48/1.46, in order
to couple to optical fibers, in order to interact with optical
fibers in coupler-half based devices, or otherwise to index match
desirable silicon dioxide (silica) claddings such as may be formed
on silicon substrates by known methods of deposition. However, in
other devices where direct contact with silica or optical fibers is
not contemplated, higher index H-PDLC in the range of N=1.53 or
higher, may be utilized, this being a range for coupling and
interfacing with other polymer waveguides.
[0092] Switchable Waveguide Grating.
[0093] FIGS. 4a and 4b illustrate the waveguide Bragg grating which
is the basic building block of all the planar designs hereafter to
be considered. This simple ESBG, connected as a transmission
grating in an integrated structure, has a bus or waveguide 31, only
the core 12 of which is shown, embedded in a substrate 33 of for
example silicon, and has an ESBG 12 formed in or mounted to its
core. An electric signal is applied to the ESBG through electrodes
35. FIG. 4b illustrates the situation when no voltage is applied to
the ESBG, resulting in the ESBG having an index free of spatial
periodic variations, which substantially matches that of the
waveguide, so that all channels of a WDM signal applied to
waveguide 31 passes through waveguide 31 without change. In FIG.
4a, when a suitable voltage is applied to ESBG 12, the grating
alters the effective index of the waveguide for at least one
wavelength of the incoming signal, illustrated as .lambda..sub.2 in
FIG. 4a, causing this signal to be reflected or dropped from the
transmission at the ESBG, while the remaining channels of the
incoming signal pass undisturbed. The channel or wavelength which
is reflected or dropped will vary as a function of the period
.LAMBDA. of the ESBG grating.
[0094] The fabrication process for such an ESBG generally involves
depositing the grating upon a silicon micro-optical structure using
one of several processes. On such process of fabrication involves
depositing a H-PDLC liquid precursor solution by application onto
silicon that contains a relief groove or V groove, and which has
been further prepared by oxidation to form a silicon dioxide
optical cladding for the waveguide. Lower electrodes may also have
been deposited on the silicon, or else the silicon itself,
conductively doped, may serve as the common lower electrode. The
H-PDLC is then polymerized by holographic lithography using
interfering laser beams (whose wavelength may be visible or UV
depending upon the chemistry of the chosen H-PDLC variant), or else
by a single laser beam together with a binary phase mask, as is
known in the art of fiber Bragg gratings. Following polymerization,
a mask and etching process removes the H-PDLC from all regions not
constituting the ESBG section. The passive waveguiding sections are
then filled in by spincoating with a second, passive polymer, index
matched to the H-PDLC.
[0095] Whereas this process is useful for H-PDLC whose index nearly
matches and is slightly greater than silicon dioxide (N=1.44), a
different process is appropriate for H-PDLC formulations whose
index may be higher, such as 1.53. In this case, after application
of a lower conductive electrode, the cladding should also be an
appropriate polymer of slightly lower index. In this process
alternative, the silicon forms a mechanical planar substrate
without grooves. Several polymer films may be deposited
sequentially, first a cladding layer, followed by H-PDLC layer,
followed by masking and then by holographic polymerization to form
the grating. Etching is then performed to form a ridge waveguide,
followed by spincoating an upper cladding layer, and masking and
depositing an upper electrode region formed by metallic or
transparent conducting film.
[0096] In all the designs for integrated optical circuits to
follow, a multiplicity of ESBGs are formed simultaneously simply by
extending the above processes to masks representing the required
number of ESBGs, with the additional possibility that different
H-PDLC formulations, different grating periods and orientations may
be applied to various individual elements within one substrate and
thereby forming an integrated device with subelements of various
individual properties. However, the inventions herein described are
based on optical designs for optimum use of H-PDLC in ESBGs, and
should not be considered to be limited or restricted by a
particular fabrication process.
[0097] The following sections elaborate on the basic building block
described above by describing functional devices that combine two
or more waveguides, one or more of which may be associated with
ESBGs.
[0098] Switchable Add/Drop Filter (SADF)
[0099] FIG. 5a shows a preferred configuration for a switchable
add/drop single channel filter. The basic functionality is to
control the adding or dropping of a specified channel between for
example an optical bus and a local or drop waveguide. The input
signal (comprising a multiplicity of independent wavelength
channels) propagates in the bus waveguide from left to right as
drawn; the grating coupled drop channel is transferred to the
counterprogagating direction (right to left as drawn) in the drop
waveguide.
[0100] The bus waveguide and drop waveguide are nonidentical,
(i.e., they differ in propagation constants) to the degree that no
significant synchronous coupling will occur by mere proximity of
the guides, unless via the mechanism of grating assisted coupling.
This nonidenticality may have consequences for packaging the
devices, in that butt coupling to single mode optical fibers will
be relatively more efficient for one of the two waveguides than the
other due to mode-matching considerations.
[0101] When the filter is switched out of the circuit by
suppressing the spatial index modulation constituting the grating
in the ESBG region, the component is transparent to all channels.
Key performance criteria for telecommunications applications
include large on/off ratio .gtoreq.35 dB, bandpass characteristics
comparable to well-designed fiber gratings, including apodization
for suppressed sidelobes, low polarization-dependent loss,
switched-off insertion loss <0.3 dB for filtered and adjacent
channels, and either zero power latchability or low power, low
drift power-on state.
[0102] FIG. 5a illustrates a substrate 33 having a pair of
waveguides 31a, 31b which are separated by distance which is more
or less comparable to the width of the cores and within the
evanescent regions of the waveguides. Thus, the evanescent regions
of the two waveguides overlap. For purposes of illustration, an
ESBG or grating is shown in or on the core of waveguide 31a;
however, the ESBG could be in the core of either waveguide or in
the evanescent region therebetween.
[0103] As shown schematically in FIG. 5b, there are a number of
coupling mechanisms for light which, for purposes of illustration,
is shown as entering at port 3 on the left side of waveguide 31b.
In the absence of coupling at a wavelength contained in the input,
the input signal or light appearing at port 3 will be transmitted
through waveguide 31b and will exit through port 1 on the right
hand side of this waveguide. By understanding and computationally
evaluating various coupling mechanisms, the desirable channel
add/drop process can be enhanced and undesirable couplings, which
would not be affected by switching of the ESBG, can be
minimized.
[0104] The second coupling mechanism is referred to as the exchange
Bragg mechanism and will occur for a wavelength .lambda..sub.j
determined by the period .LAMBDA. of the ESBG grating. In
particular, the grating couples out wavelength channel
.lambda..sub.j from the WDM signal on bus 31b and sends it in the
opposite direction down waveguide 31a toward port 4. The grating
period .LAMBDA. in order for an exchange Bragg coupling to occur
for wavelength .lambda..sub.j is determined 2 = j n a + n b Eq . (
4 )
[0105] where n.sub.a is the effective index of waveguide 31a and
n.sub.b is the effective index of waveguide 31b. The effective
index of a waveguide is the modal index of the waveguide based on
the indexes of both its core and cladding regions and the modal
field solution. The indexes for both waveguides are evaluated at
.lambda..sub.j. The bandwidth of the filter response is given by 3
= 2 j 2 ( n a + n b ) Eq . ( 5 )
[0106] where .kappa. is the coupling coefficient which can be
determined using well-known methods of coupled mode approximation
computations. .kappa. can be controlled by the grating index
contrast and, in ESBGs, may be tuned electro-optically. Side lobe
responses often occur outside the main channel bandwidth for
exchange Bragg coupling. These can be suppressed by apodizing the
grating coefficient .kappa. in a variety of ways. A simple way to
apodize .kappa. is to increase the distance between the two
waveguides away from the center of the device as shown in FIG. 5a,
or in other words to curve the ESBG with the center closest to the
bus waveguide and the ends further away.
[0107] In addition to the exchange Bragg coupling, there is also
evanescent and direct Bragg coupling, both of which can interfere
with and degrade the ideal spectral response. Evanescent coupling
occurs when the modal indices of the two waveguides are
substantially equal so that light passes between the two waveguides
in a somewhat unregulated fashion, resulting in the drop
wavelength, for example .lambda.1 in FIG. 5a, also being output at
port 2 in addition to port 4. .lambda.2 and .lambda.3 may also pass
to port 2 as a result of this action. This coupling is undesirable
because it would not be substantially altered by switching the
ESBG, and therefore would contribute to crosstalk. As shown in FIG.
5b, direct Bragg reflections also degrade the ideal spectral
response by causing some of the drop or exchange wavelength, for
example .lambda.1, to be reflected in waveguide 31a, for example as
a result of the action of ESBG 12. Direct Bragg coupling can be
quite large and cannot be eliminated completely. However, because
it is a phase-matched process, one way to reduce its impact is to
make the phase-matching wavelengths for the exchange Bragg and
direct Bragg mechanism very different. This is accomplished by
making the index values for each of the waveguides 31a, 31b
different. In the spectral domain, both Bragg mechanisms lead to
stop bands. An essential condition to assure that the coupling
mechanisms do not strongly interfere is that the stop bands should
not overlap in the wavelength domain. For this condition,
.vertline.n.sub.b-n.sub.a.vertline..gtoreq..vertline..DELTA.n.vertline.
Eq. (6)
[0108] where .DELTA.n is the index contrast of the grating, this
being the differences in the index spatially along the grating when
the grating is powered. The larger the difference between the
effective indices of the two waveguides, the less adverse effect
direct Bragg will have on filter performance and the more ideal the
filter or drop characteristics of the component will be to for
example drop the desired wavelength from the signal on waveguide
31b with minimum loss as a result of the transfer. Fortuitously,
since evanescent coupling is also caused by an index match between
the waveguides, varying the effective index of the two waveguides
also substantially eliminates evanescent coupling.
[0109] Thus, the maximum filter bandwidth depends on the index
contrast .DELTA.n of the ESBG and on the separation between the two
filter waveguides. The bandwidth scales linearly with grating index
contrast. For an ESBG spatial index modulation parameter of
.DELTA.n=0.01, a typical value for index contrast, and a waveguide
to waveguide separation of 1 micrometer, the bandwidth can be as
large as 1 nm. This is sufficient for WDM applications. The
bandwidth can be made larger by linearly chirping the grating
period, or in other words by making slight spatial changes in the
grating period.
[0110] For small enough wavelength, the ESBG grating can
phase-match radiation modes and this leads to power loss. For the
exchange-Bragg filters designed in accordance with the foregoing
rule that the effective index of the two waveguides differ by at
least .DELTA.n, the radiation-matching regions are those
wavelengths which are 5 .mu.m and smaller than the exchange Bragg
center wavelength. For high index ESBGs with an index of about
1.53, these wavelengths would be displaced from the center
wavelength by 50 nm. The degree of radiation loss can be minimized
by putting the grating only on the filter waveguide, and making the
filter waveguide higher index than the input bus waveguide.
[0111] FIG. 5c shows the simulated optical performance using
fabrication parameters as follows: bus waveguide width=8 .mu.m,
N=1.4492, drop waveguide width=4 .mu.m, N=1.53, length=10 mm,
grating period=523 nm, ESBG spatial index modulation=0.01, minimum
guide separation=3 .mu.m. Ports 1, 2, 3, 4 refer to the
corresponding ports of FIG. 5b. In this example, no apodization was
applied. In FIG. 5d, the benefits of apodization of the coupling by
the curved interface between the waveguides are shown.
[0112] Based on modeling of this kind, the design shown in FIG. 5a
has been found to have nearly optimum filter shape, with a box-like
drop bandwidth and side lobes suppressed by apodization. Also, the
device is not highly sensitive to length, as the spectral response
shape remains similar if the length is increased to maximize the
dropped power fraction. Drawbacks of such a filter are that a
narrower bandwidth requires a longer device, which is not true of
alternative, resonator-type designs to be disclosed below. Also,
power is dropped in the oppositely propagating direction, and the
design requires that the two waveguides by mismatched (i.e., that
their propagation constants are substantially unequal).
[0113] FIG. 5e shows an approach to integrated planar designs that
extend the single element SADF to a four channel array that drops
(or if used in the reversed direction, with inputs replaced by
outputs and vice versa, adds) any of four wavelength channels,
independently, into the drop port. Again, the bus and drop
waveguides are nonidentical, limiting the practicality of this
design for WSXC. In practice, limits to scalability depend on
losses at each node, which accumulate as the array is extended to
address a greater multiplicity of channels.
[0114] However, a practical problem with the two waveguides
differing by .DELTA.n is that the core dimensions of the higher
index waveguide must be made smaller than the diameter of an
optical fiber in order for it to remain single mode. This causes
coupling loss when butt coupling to fiber pigtails. For small index
ESBGs (core index of 1.47, cladding index 1.444), the square core
width is 4 micrometers and the resulting butt coupling loss can be
about 20 percent. For large index ESBGs (core index 1.53, cladding
index 1.444), the insertion loss can still be made a modest 30
percent if the core dimension is reduced to 1 micrometer. However,
a 2 micrometer square core width in this case could give a 60
percent insertion loss, which would be unacceptable, and is
therefore not utilized.
[0115] FIG. 5f illustrates a planar integrated 2.times.2 space
switch which functions in a manner substantially the same as the
2.times.2 space switch shown in FIG. 21 and described in
conjunction with this figure. FIG. 5f illustrates that such a
switch may also be fabricated utilizing planar integration.
[0116] Wavelength Selective Crossconnect (WSXC)
[0117] The function of a WSXC is closely related to SADF, except it
is configured to interconnect two or more optical buses instead of
a bus distributed to a local node. In general, a WSXC selectively
exchanges M-channel WDM signals among N.times.N incoming/outgoing
optical fibers in order to dynamically reconfigure and route
traffic within architectures that regard each wavelength as a
separate and independent "virtual fiber." An N.times.N.times.M WSXC
connects N fibers and M wavelengths. Future networks will rely on
such a device in order for WDM to realize its full potential to
multiply network capacity. Based on industry plan, devices ranging
in scale from 2.times.2.times.4 up to 128.times.128.times.80 or
larger may be required.
[0118] Several approaches to WXSC are being actively pursued in a
number of laboratories; among these are INP integrated WDM routers
combined with phase shifters or space switches, modular passive WGR
routers coupled to thermo-optic or other switches, and
acousto-optic filters. No leading technology has been established
as yet, and most existing approaches are costly.
[0119] In accordance with this invention, a single switching point
of a WXSC is a four port active switch with two input and two
output ports. The switch is activated electro-optically, by the
ESBG mechanism of toggling the periodic spatial index modulation
constituting the grating for each selected element, on or off. By
programming the control voltages for each ESBG element on a unified
chip or substrate containing a multiplicity of such elements, a
very large number of switching states may be realized for rerouting
and reconfiguring the exchange of WDM signals.
[0120] FIGS. 6a and 6b illustrate two variants of elementary
cross-point switches based on ESBGs. Each of two main embodiments
uses two ESBGs per element. The function of an individual element
is to embody the elementary wavelength selective cross-point
function. When the switch is not active, all wavelength channels at
a particular input port will pass unaffected to a particular output
port. When the switch is activated, a particular wavelength channel
(if present) at each input port will be crossconnected to the
alternate output port. All non-selected wavelengths remain
unaffected.
[0121] In FIGS. 6a and 6b, unlike the earlier SADF devices, the two
bus waveguides are identical (have equal propagation constants),
and only the internal "s" or "ring" guides have substantially
different propagation constants. Thus these also provide more
efficient coupling to optical fibers.
[0122] Referring first to FIG. 6a, an ESBG 12a is in the evanescent
region of waveguide 31a and a grating 12b is in the evanescent
region of waveguide 31b. The waveguides are sufficiently separated
so that their evanescent regions do not overlap. ESBGs 12a and 12b
are connected by a curved or "s" waveguide 37. With this
configuration, waveguides 31a and 31b may be of identical size and
material and may have the same index, thus significantly
simplifying fabrication and permitting standard couplings to be
used for all waveguides so as to minimize insertion loss. However,
undesired couplings (i.e. evanescent and direct Bragg couplings)
are significantly suppressed by having waveguide 37 much smaller
than the waveguides 31 so as to have an index differing from that
of the waveguides 31 by an amount significantly in excess of
.DELTA.n. The configuration of FIG. 6a thus provides superior
suppression of undesired couplings and improve insertion loss
performance, but is more complex and expensive than the
configuration shown in FIG. 5a. While the gratings in FIG. 6a are
not shown as apodized to reduce side-bands, this or other
techniques could also be utilized with these embodiments for such
side-band reduction.
[0123] FIG. 6b differs from FIG. 6a in that, instead of only a
single waveguide 37 interconnecting ESBGs 12a and 12b, a pair of
waveguides 37a, 37b are provided to perform this function, the ESBG
12 and waveguides 37 forming a closed ring 39. An advantage of the
ring configuration of FIG. 6b over that shown in FIG. 6a is that,
whereas with the configuration shown in FIG. 6a signals traveling
between the waveguides 31 in either direction all travel through
the same waveguide section 37, raising the potential for cross
talk, in the configuration of FIG. 6b, signals from input 1 on
waveguide 31b travel through waveguide segment 37a while signals
from input 2 on waveguide 31a travel through waveguide segment 37b,
reducing the potential for crosstalk. Another, perhaps more
significant advantage, is that, whereas for the embodiment of FIG.
6a cross-connected and unaffected signals travel in opposite
directions on a waveguide 31, for the embodiment of FIG. 6b all
signals propagate in the same direction within any waveguide. The
design of FIG. 6b, whereby the drop signals propagate parallel with
the inputs, is considered to be more convenient for large scale
integration, connectorization, and interfacing with the other
elements of the optical network. The design of FIG. 6b is therefore
the currently preferred design, although the design of FIG. 6a may
be preferred for selected applications.
[0124] The configurations of FIG. 6a, 6b are further advantageous
in that, with two ESBGs at the coupling, wavelength selectivity can
be enhanced in that each signal is doubly filtered. As a result,
sidebands can be suppressed by up to 50 dB, which is not possible
with a single ESBG.
[0125] FIG. 7a compares the filter response of the S crossconnect
(solid line) with a single exchange Bragg filter of the type shown
in FIG. 5a (dashed line). (Here the S ESBGs have also been curved
so as to effectively apodize the response.) The response of the
S-crossconnect is virtually the squared response of the exchange
Bragg filter. Thus the sidelobes are considerably reduced while the
flat passband is preserved. FIG. 7b shows the details of the
throughput response, where inband crosstalk can be less than -30
dB. In crossconnects it is vital to fully extract the dropped
wavelength from the input bus, because a new signal at the same
wavelength will be re-injected, and so-called inband crosstalk must
be minimized. The inband crosstalk can be further improved by
increasing the grating reflectivity, either by making the spatial
index modulation of the ESBG larger, by raising the average index
of the ESBG region or by making the ESBG longer. The freedom to
adjust these parameters by the use of H-PDLC is another advantage
of this material family for optimizing these designs.
[0126] FIG. 7c shows a computed typical optical response of the
ring crosspoint 39. This is similar to the S-crossconnect, except
that spurious spikes are observed related to power trapping in the
ring. In addition, the inband response is somewhat compromised
because of the ring resonances. While the two ESBGs shown in the
"s" form of FIG. 6a for the "ring" form of FIG. 6b could in
principle be electrically controlled independently, it is intended
in these designs that the ESBGs within each "s" or within each ring
be electronically switched simultaneously and together in order to
perform the described cross-connect function.
[0127] While the FIG. 5a configuration has been identified
primarily as an SADF, and the FIG. 6a, 6b configurations primarily
as WSXC, in fact both could be used for SADF. From this point of
view, for the components shown in FIGS. 5 and 6, the component of
FIG. 5a is advantageous in that it can provide a box-like response,
side lobes being easily reducible by apodization, that it is fairly
length insensitive and that it is fairly simple and
straight-forward to fabricate. However, since it required that the
waveguides be mismatched in order to achieve useful performance,
fabrication can become more difficult and there is a potential for
insertion coupling loss problems. The embodiments of FIGS. 6a and
6b, while more complex, may be easier to fabricate since the
waveguides 31 can be made of uniform size and undesired coupling
modes are more easily and effectively suppressed. Where the device
is being used only as a drop filter (i.e., a multichannel WDM
signal on for example bus 31b, which may for example be a main
transmission bus, has a single channel transferred to an
appropriate local bus 31a), the configuration of FIG. 6a may be
preferred. However, where the component is being used as both an
add and drop filter, a WSXC or SADF (i.e., in addition to the
channel being transferred from main bus 31b to local bus 31a,
signals on bus 31a of the desired waveguide are also transferred
through gratings 12 and waveguide 37 to bus 31b), the configuration
of FIG. 6b may be preferred so as to minimize potential cross
talk.
[0128] FIGS. 8a, and 8b are diagrams of illustrative cross-connect
arrangements between multiple waveguides for multiple wavelengths.
FIG. 8a for example shows two waveguides 31a, 31b each of which may
carry four channels or wavelengths. At the junction of these two
waveguides, four transfer rings 39.sub.1-39.sub.4 are provided,
which rings are each the same as the ring 39 shown in FIG. 6b, each
of which rings has an ESBG grating period which causes the ring to
perform the add/drop function for a specific corresponding
wavelength channel 1-4. Thus, by selectively energizing one or more
of the rings 39, information on selected channels may be added,
dropped, or transferred between the waveguides 31. This is
therefore a design for a two fiber in, two fiber out ESBG based
WSXC, with four wavelengths (i.e., a 2.times.2.times.4 network).
Rings 39.sub.1-39.sub.4 could also be used to transfer between a
main bus and local buses, each accepting a limited subset to the
wavelengths on the main bus.
[0129] FIG. 8b shows a more complex network in the form of a Benesh
net of six, 3-wavelength exchange sections connecting four input
and four output waveguides, for what is sometimes referred to a
4.times.4.times.3 configuration. This embodiment functions in a way
similar to that of the embodiment shown in FIG. 8a to permit the
transfer of any of the four channels inputted on any of the four
waveguides to be outputted on any of the four waveguides.
[0130] The network designs shown in FIGS. 8a-8b are merely
illustrative of possible design options for constructing networks
in accordance with the teachings of this invention and networks
adapted for any number of input and output fibers and to any number
of wavelength channels can be constructed using these or other
network configurations, including the "s" form of FIG. 6a. The use
of the Benesh net in FIG. 8b for the interconnections, which has
the advantage of being rearrangably nonblocking, is know in the art
as an efficient nonblocking architecture, but is by no means the
only possible architecture offering this advantage, it being only
one of the network architectures known to the art which might be
applied for interconnecting such ESBG wavelength exchange regions
into a multifiber network. A survey of network architectures may be
found in H. S. Hinton, An Introduction to Photonic Switching
Fabrics, Plenum Press, 1993.
[0131] Obviously, the complexity of the coupling increases as the
number of waveguides and channels increases; however, networks
employing the ESBG technology of this invention, including the
illustrative network design do provide substantially scalability
and, since, as will be discussed later, the entire device can be
fabricated using fairly straight-forward integrated circuit
technology, with the result that significant increases in the size
and complexity of the network do not result in corresponding
increases in cost. This is another significant advantage of the
technology. Thus, the advantages of ESBG based add/drop and WSXC
devices include on-chip intergratebility, polymer manufacturing
capability and low-cost, single-step grating fabrication, coupled
with high channel selectively, low cross talk and insertion loss
and fast switching speeds, in the tens of microsecond range.
Perhaps most important, the flexible chemistry and fabrication of
ESBGs permits a number of differently optimized elements, differing
in index, grating characteristics, period, orientation, and other
parameters, to be fabricated together on a single substrate, which
is difficult if not impossible to accomplish using any materials
technology other tha H-PDLC.
[0132] Resonator Devices
[0133] Resonators are still another waveguide and grating
configuration which may be utilized for implementation of SADF and
WSXC element. Resonators are defined for purposes of this
application as grating-based waveguide structures wherein storage
of optical energy in a localized region is a significant principle
of device function.
[0134] Resonator based wavelength filters have several
characteristics that are superior to filters based on other
mechanisms. These characteristics include the ability to synthesize
arbitrary filter shapes by cascade-coupling numerous resonators
and, unlike interference filters, the ability to control pass
bandwidth by techniques other than changing device length.
Resonators also do not have troublesome out-of-band side lobes.
Instead, the out-of-band response decreases monotonically at a rate
determined by the number of resonators comprising the channel drop
filter.
[0135] Resonators can be realized by use of Bragg gratings
incorporating a quarter waveshift, patterned on or near a
wavelength, as depicted for the waveguide 60 in FIG. 9. The quarter
waveshift 62 acts as a cavity while the gratings 63 to either side
serve as distributed mirrors. The optical energy at a selected
wavelength is trapped in the waveguide region 64 of the quarter
waveshift section and circulates resonantly (wave 65). The period
of the grating is determined by 4 = j 2 n e Eq . ( 7 )
[0136] where n.sub.e is the effective index of the mode in the
resonated waveguide evaluated at .lambda..sub.j.
[0137] Wavelength selected channel dropping is accomplished by side
coupling the resonator to one or more bus waveguides. There are
several ways in which this may be accomplished, as depicted by
FIGS. 10a-10c. In all case two resonators are required to get 100
percent extraction of the desired channel. This second resonator,
which is referred to as the reflection resonator, is required in
order to cancel the backward wave generated by the first resonator,
which is referred to as the drop resonator.
[0138] In the resonator filter of FIG. 10a, the filter output drop,
port 2, is directly coupled to the drop resonator 68. The reflector
resonator 70 is coupled to the bus waveguide 72, but displaced
spatially from the quarter-wavesection 62 of resonator 68 by an
integer number of wavelengths, plus a half wavelength. While the
filter output is dropped in the forward direction in FIG. 10a, it
can also be dropped in the reverse direction by attaching the drop
port 2 to the opposite end of drop resonator 68. The unterminated
ends of the grating sections (port 4) must be made long enough so
that no power escapes through them. Of multiple wavelengths
inputted at port 3, one will be dropped to drop port 2 while the
remaining wavelengths will pass unaffected to transmission port 1.
The length of grating 68 and its index contrast between
quarter-wavesection 62 and drop port 2, determines the bandwidth of
the device.
[0139] In the structure of FIG. 10a and other resonator
configurations, it is understood that the ESBGs constituting the
several grating components are all to be switched on or off in
concert, either by coordinating their respective electrode-applied
signals, or by fabricating a single monolithic electrode over the
entire resonator portion constituting multiple ESBGs.
[0140] FIGS. 10b-10d are three examples of higher order filters
which give improved filter performance by coupling multiple
resonators. The channel drop filter in FIG. 10b is similar to that
in FIG. 10a, a bus waveguide 72 being coupled to a series of drop
resonators 68' and also to an identical series of reflector
resonators 70'. Each resonator is coupled to its neighbors by a
section of the corresponding grating. The length L2 of the
connecting gratings between the quarter waveshifts 62 determines
the details of the filter shape.
[0141] The channel drop filter of FIG. 10c has similar performance
to that shown in FIG. 10b, differing from the filter shown in FIG.
10b in that the coupled drop resonators 68a, 68b and the coupled
reflector resonators 70a, 70b are stacked adjacent to one another
instead of being in series. This has the advantage of keeping the
device length shorter. The distance between the portions of each
coupled resonator determines the details of the filter shape.
[0142] While in the discussion above, the resonator waveguides 74
and the bus waveguides 72 have been matched, this is not a
requirement. The main difficulty in using mismatched guides is that
the power transfer efficiency from the input bus to the resonator
decreases, the effect being similar to a synchronous directional
coupler. Thus, it is generally more difficult to realize large
linewidths unless the bus and resonator waveguides are very
strongly coupled and/or the grating index contrast is large. In
addition, because the modes walk-off in space due to the difference
in propagation constants, higher order filters can only be realized
by series coupled resonators as depicted in FIG. 10c. However, such
devices may be preferable for ESBGs utilizing relatively high index
composites, making it desirable to fiber match the bus
waveguides.
[0143] Three guide-channel dropping filters are schematically
illustrated in FIGS. 10d-10f. In these embodiments, the resonator
is side-coupled to both an input and an output bus (i.e., the
resonator is in the cladding between the buses, which claddings
overlap). For channel dropping filter embodiments of such
resonators, two resonators are still required. The primary
advantage of the configurations shown in FIGS. 10d-10f is that the
resonator is independent of the input or output waveguides. In
addition, for the embodiments of FIGS. 10e and 10f, the waveguide
separation S and the resonator lengths L are independent design
parameters, unlike the situation for the two-guide structure where
these parameters are related. In FIG. 10d, the two resonators are
coupled together. When the lengths of all grating sections are
appropriately chosen, power is dropped into port 2 at resonance. In
the embodiment of FIG. 10e, the two resonators are decoupled.
However, a phase delay section 78 must be introduced between each
resonator along the bus waveguides. This phase delay may be
realized by making the input and output bus waveguides different in
length or by changing the waveguide dimensions to modify the
propagation constants. For matched waveguides, the phase delay of
the input bus must amount to .+-..pi./2, while the delay of the
output bus must amount to .pi./2. For this embodiment, power is
dropped to port 4 at resonance. This embodiment is less restrictive
than the one appearing in FIG. 10d, because the resonator lengths,
and the bus-to-resonator separation, are independent design
parameters.
[0144] Stated another way, FIG. 10e shows a resonator channel
dropping filter using unperturbed waveguides 72, 74 with a split
resonator 76a, 76b placed between them. The split resonator is
required in order to achieve full signal extraction. The advantage
of this configuration is that the input and the filtered bus
waveguides can be optimized for in/out fiber coupling, while the
resonators can be independently optimized to achieve low loss and
other advantages. Phase delay section 78 must be inserted along the
bus waveguides in between the two resonators. The phase delay
between quarter-wavesections of each resonator must amount to
.+-..pi./2 in the input bus 72 and .pi./2 in the output bus 74. In
either case, multiples of 2.pi. phase may be added to the waveguide
delays without affecting the performance.
[0145] The channel drop filter of FIG. 10f is similar to that shown
in FIG. 10e except that multiple parallel-coupled resonators are
provided between the buses as for the embodiment of FIG. 10c. The
multiple coupled resonators could also be connected in series as in
FIG. 10b rather than in parallel. As for the embodiment of FIG.
10e, delay sections 78 are required. What has been said for
previous embodiments concerning multiple resonators connected
either in series or in parallel applies also for the embodiment of
FIG. 10f.
[0146] FIG. 11 shows one example of the response, in this case from
a matched bus/resonator, for the power out of the various ports as
a function of deviation from channel center wavelength.
[0147] Planar Integration
[0148] In order to integrate a number of different elements of the
above-described categories on a single substrate, it is desirable
to employ structures based on planar-waveguide based integration,
as previously discussed and as will be illustrated for a more
complex structure in FIGS. 12a-12e and 13a-13b. One path to such
integration relies on an extension of the fabrication process
described earlier, wherein using silicon micro-optical bench
technology, waveguides are comprised of regions of high index
(doped silica or polymer) deposited in pure silica claddings which
in turn are processed in etched silicon substrates manufactured by
well established methods of the microelectronics industry.
[0149] Thus one path of multidevice integration is to extend such
fabrication to more complex networks of devices and elements. FIGS.
12a-12e show a straightforward extension of the fabrication method
to encompass combinations of disparate devices. The waveguide
structures 48 is first manufactured. ITO or other electrodes 20 are
deposited where required (FIG. 12a). Then a H-PDLC film 30 is
applied to the structure by spincoating or other polymer coating
techniques (FIG. 12b). A binary phase mask 52 is then put in place
which contains pre-designed grating parameters in order to produce
the various periods and directions of the required gratings, to
perform the differing functions described (FIG. 12c). FIG. 12d
shows the cover plate (for example glass or silica) with the upper
electrodes formed thereon being mounted to the substrate, and FIG.
12e shows the final assembly.
[0150] A different path to multidevice integration is disclosed in
FIGS. 13a-13b. As discussed earlier, ESBGs may be located in either
the core or cladding regions of planar waveguides. Although
generally the core location results in stronger interactions and
better performance, some advantages apply to cladding located
ESBGs, which may also be termed "overlay" ESBGs in that the grating
films overlay a structure of underlying waveguides.
[0151] In particular, overlay ESBGs open the possibility for a
large scale integration technology in which a monolithic ESBG film
is sandwiched between an upper network of waveguides and a lower
network of waveguides. The overlay approach can be transferred to
an integrated planar waveguide technology by the sandwich technique
illustrated in FIGS. 13a and 13b. The lower layer 46 is a silicon
based structure containing many silica based or polymer waveguides
48 in a network that approach the surface in selected interaction
regions. A matching upper silicon layer 46' contains a second
network of waveguides 48'. The design is such that the waveguides
from the upper plane contact or cross or coincide with the
waveguides from the lower plane at selected loci or nodes 50, and
otherwise are fully clad so as to protect low loss propagation.
[0152] Where waveguides from the upper section have exposed
evanescent regions crossing exposed waveguides from the lower
section at a substantial angle between the two, an ESBG film 12
placed between them can couple them in the 2.times.2 space switch
44 to be described later using fiber based devices, the principle
being the same. Either one of the substrates may have the H-PDLC
film formed thereon, with the ESBG gratings formed therein in
manners discussed elsewhere, or an H-PDLC film with ESBGs formed
therein, or floated ESBG decals, may be mounted between the
substrates. Electrodes are preferably formed on the substrates at
each node.
[0153] Where waveguides from the upper section have exposed
evanescent regions crossing exposed waveguides from the lower
section in a parallel path, an ESBG film placed in one of the
waveguides or between them as previously described to implement
SADF, WSXC, other couplers or other devices described earlier,
provided that the upper and lower waveguides are identical or
nonidentical in the coupling region, as required, which may be
implemented by well known methods of channel waveguide design.
[0154] By combining such individual devices into networks, an
unlimited variety of integrated device architectures can be
fabricated in which the individual switching, path-exchange, and
wavelength selective properties can be combined and permuted in
many architectures.
[0155] Finally, the waveguide to free space principle described
later can be implemented provided there are holes in the silicon
layers for light to be coupled in free space out of or into the
waveguides. This could be used to provide interplane communication,
in order to extend the sandwich structure into multiple layers.
[0156] Single Step Holographic Exposure of Multiple and Diverse
Overlay Nodes
[0157] In a planar integration geometry as described, the middle of
the sandwich is a collection of overlay ESBG films comprising the
various individual gratings, with different orientations and
periods (grating vectors) but with a common layer thickness. An
efficient one step manufacturing process will result if all such
gratings required for the multiple functions of the various nodes
are made simultaneously.
[0158] A binary phase mask, well known to optical science, will
produce two beams when side-illuminated by a single laser beam
(this being the H-PDLC exposing beam, typically 488 nm). These two
beams then cross in close proximity to the polymer film, producing
ESBGs. After exposure, the finished lower (or upper) plane with
ESBG active films in place is removed from the fixture containing
the binary multi-mask. Thus one step of holographically exposure
will simultaneously produce a number, possibly hundreds, of device
nodes on a single planar chip, several of which may have differing
characteristics from the others, which device nodes may be of a
single type or may be of a variety of different types. After
stabilization, the two halves are combined and finally
packaged.
[0159] Coupler-Half Devices
[0160] A device also suitable for practicing the teachings of the
invention, which device has been known to the art of fiber optics
since the early 1980s, is the polished coupler or coupler-half
(also known as a side polished fiber). A single mode optical fiber
is bent on a long radius (such as 15-50 cm more or less) and
cemented into a groove in a silica block, which is used to hold and
support it while the fiber is polished to expose an oval region of
cladding close to the core. The function of the glass block is
primarily to hold the fiber, but it also provides a smooth flat
surface coinciding with the polished fiber facet. With a deep
enough polish depth (for example, to within 2 .mu.m of an 8 .mu.m
diameter core), substantial intensity of the evanescent optical
field becomes accessible. As is commonly practiced, such
coupler-half devices can be used as a variable splitting ratio
coupler by mating their polished surfaces. Also, coupler-halves
have been used as substrates to couple single mode fibers into
metallic, polymer or liquid crystal films for the purpose of making
fiber optic polarizers, spectral filters, and other components.
[0161] In accordance with the teachings of this invention,
coupler-halves may also be used for low cost and efficient coupling
of single mode optical fibers to overlay type ESBG films, which are
easily deposited and supported on the polished surface, or to
certain other types of film, for example thermo-optic film. Devices
based on optical fibers are simple to make and, unlike planar
waveguide structures, automatically offer easy coupling into and
out of the optical fibers and connection to other fibers.
[0162] FIG. 14a shows the structure of such a device 32, with FIG.
14b being a chart of the indices and the dimensions of the various
components and layers for an illustrative such device, although
substantially different index profiles are also intended to be
encompassed within the invention. Known calculation approaches
based on coupled mode theory can be used to design such devices.
FIG. 14c is a sectional view along the line c-c in FIG. 14a, with
the electrodes 20 on top and bottom of ESBG 12 to provide a
vertical electric field. FIG. 14d is the same view as FIG. 14c for
an alternative embodiment where the electrodes are on the sides of
the ESBG to provide a transverse electric field. For the FIG. 14d
embodiment, the electrodes need not be transparent, gold electrodes
being used for a preferred configuration.
[0163] Switchable Drop Filter
[0164] FIG. 15 shows a single-channel (single-wavelength) filter
switch 34 utilizing the coupler-half/ESBG technology. The two
states of such a switch are, in terms of end functionality, either
to filter (remove) or not to filter one predetermined wavelength
channel from a single mode optical fiber 36 transporting a
multiplicity of such wavelength channels (for example 16 separate
and independent optical signals separated in center wavelength 0.8
nm, corresponding to 100 GHz) However, the planar waveguide SADF
described earlier, the drop port for this coupler half device is
not available for easy capture by coupling to an optical fiber, so
the device function is simply to remove one channel from the bus.
In its unpowered state, this device performs a function of
selectively filtering out one wavelength channel at a wavelength
.lambda.=2n.sub.eff.LAMBDA. (applying a large loss to that specific
channel) while passing all others with minimum or zero loss. In its
powered state, the switch passes all wavelength channels, ideally
equally and without loss. Such a switch is useful in certain WDM
applications, often in combination with other such components. For
example, a set of 16 independent parallel switches could be
constructed to deal with each of the 16 wavelength channels
separately and independently.
[0165] To fabricate the device 34 of FIG. 15, a polished coupler is
prepared using a long radius bend fiber 36 so as to provide
approximately 3-4 mm of interaction region in the form of exposed
section of the fiber proximate to the core. After the side of the
fiber is polished away, the modal field extends above the surface.
A transparent electrode 20 (see FIG. 14a) is then deposited on the
polished surface in for example the form of indium tin oxide,
thickness 100-200 nm, by methods such as magnetron sputtering.
Using spacers known to the liquid crystal display industry, such as
glass microspheres, a liquid film of H-PDLC is applied to the
surface, after which a cover glass (fused silica) also coated with
a transparent electrode, for example indium tin oxide, is applied.
Alternatively, polymer conducting electrodes may be used whose
index is more closely matched to the silica and ESBG materials,
with the advantage of lower loss and simpler design computations.
The thickness of the ESBG film may be varied from less than 1 .mu.m
to more than 10 .mu.m depending on design and indices of available
materials. The principles of waveguiding in planar films are well
understood. Considered as a single mode waveguide, the ESBG film is
characterized by a V parameter given by 5 V = 2 h ( n ESBG 2 - n
substrate 2 ) 1 / 2 Eq . ( 8 )
[0166] where h=thickness of ESBG film, .lambda.=free space
wavelength, n.sub.ESBG=average index of ESBG film and
n.sub.substrate=index of fiber cladding. As is well known, the
condition for single mode propagation is that V<3.14. Device
functioning is sensitive to the index of the ESBG film.
[0167] Very thin films are necessary for a low loss device if the
polymer film average index is relatively high, for example 1.52,
compared to the silica cladding, 1.46, in order to prevent the
polymer film from drawing excessive optical power from the fiber.
But if the polymer composite possesses an index more closely
matched to the silica cladding, as described earlier, a thicker
film may be utilized, resulting in a stronger coupling. For
example, an ESBG film 8 .mu.m thick may be utilized if the average
index can be precisely formulated and controlled so that in it's
powered state it precisely matches fused silica .apprxeq.1.46 and
in it's unpowered state it is higher by approximately 0.5%.
[0168] A grating is recorded in the liquid by holographic
polymerization using interfering beams from an external 488 nm
Argon ion laser or other suitable laser, depending upon the
photosensitizer absorption spectrum included in the solution, such
laser beams to be applied as illustrated in FIG. 16. The effect of
the grating is substantially to couple the forward propagating
light from the fiber mode to the backward propagating direction in
the ESBG film.
[0169] The required grating period .LAMBDA. is determined by the
formula .lambda.=(n.sub.eff+n.sub.fiber).LAMBDA., where .lambda. is
the desired center wavelength to be filtered and n.sub.eff the mode
index of the polymer composite film and n.sub.fiber the modal index
of the optical fiber. If for example it is desired to filter
.lambda.=1448 nm and n.sub.eff=1.47 and n.sub.fiber=1.45, then the
grating must be fabricated so that the period .LAMBDA. is precisely
493 mm. A grating of this period can be produced by exposing the
liquid H-PDLC thin film (captured between the polished coupler
surface and the glass cover plate) to 488 nm laser beam's with a
half angle such that sin .theta.=.lambda.'/2.LAMBDA., where
.lambda.' is now the wavelength of the argon laser, 488 nm. In this
example, .theta. will be about 30 degrees. If the spatial index
modulation .DELTA.n/n<0.001 and L=3000 .mu.m, then
d.lambda./.lambda.<2.5.times.10.sup.-4, which is smaller than
the channel spacing, as desired.
[0170] In its unpowered state, the filter device 34 will
selectively remove the signal at 1448.0 nm (in this example) by
dispersing it in the polymer film, while approximately uniformly
transmitting all other wavelengths, as for example 1447.2 nm and
1448.8 nm, and others. Applying an electric field to the device
between electrodes 20 will switch the ESBG film 12, meaning that
the periodic index modulation will be caused to disappear by
reorienting the liquid crystal directors inside the aspheric
microdroplets, matching the liquid crystal index to that of the
polymer host, and thereby negating the grating effect. This in turn
suppresses the filtering and allows the channel in question to now
propagate along with the other channel signals. This switching of a
single wavelength within the multiplicity of wavelengths is
sometimes referred to as a single-channel drop switch.
[0171] Performance measures for such a switch include the dynamic
range (on/off ratio) for the affected channel, the shape and width
of the spectral response characteristic, the insertion loss for all
other channels in the powered and unpowered states, temperature
sensitivity of these properties, voltage of operation, speed of
operation, power consumption, and polarization sensitivity.
Depending on the thickness of the ESBG film, its index and the
thicknesses and indices of the transparent electrodes, such a
device may be highly polarization sensitive in its filtering
properties, losses, dynamic range and other operating
characteristics, but such polarization sensitivity can be minimized
or eliminated by sing H-PDLC formulations which closely index match
the optical fiber core-cladding range.
[0172] Switchable Outcoupler
[0173] FIG. 17 shows a device 38 fabricated similarly to the Bragg
filter but designed as a switchable grating coupler. A grating
coupler, is a device known to fiber optics since the early 1970's,
uses a grating to couple light propagating from free space into a
single mode optical fiber 36, or equivalently in the other
direction from the fiber into free space. Such a device is useful,
for example, to direct a sampled part of a guided wave to an
external detector or array of detectors, or for otherwise coupling
optical power from a fiber into a free space distribution in order
to interconnect with computer processors, or in complex switching
systems. While design of an overlay grating for this purpose is
known to the art of fiber optics and waveguide technology, such
devices have not previously been switchable. The operational
difference from the drop switched filter 34 of FIG. 15 is simply
that the period of the grating is somewhat longer, designed to
satisfy the condition (in the simplest case, first order diffracted
mode) 6 n eff - sin = Eq . ( 9 )
[0174] where .theta. is the angle of the radiated beam from the
normal to the planar surface. As an example, for .lambda.=1550 nm
and n.sub.eff=1.47, a grating period .LAMBDA.=1200 nm will
outcouple (or incouple) light from the guided mode to (or from)
free space at an angle .theta.=10.2.degree. from the normal. It is
apparent that this outcoupling angle will be wavelength dependent,
and in this example will vary from 10.2.degree. to 5.45.degree.
over the wavelength range 1550-1650 nm. This variation can be
applied to outcouple various wavelength channels to different
positions or detectors in free space constituting a type of
spectrometer. Thus a multiwavelength WDM signal propagating in the
fiber will be outcoupled into free space as a fan-shaped array of
signals, with each channel propagating in a slightly different
direction. Switching such a device could be used as a WDM signal
sampling spectrometer, to divide the signals in space to monitor
the equality of optical power in the various channels, or in
general as a diagnostic tool for monitoring the performance and
status of multiwavelength networks.
[0175] By means of applying an electric field through electrodes 20
(not shown, but same as for FIG. 15) to suppress the grating
spatial index modulation, the coupling between radiated and guided
modes can be disabled. In the powered state of the switch, light
propagating in the fiber will then substantially continue through
the device and exit the fiber at its far end, ideally without loss.
In this device, the switching effect is therefore to couple or
decouple guided and free space modes. The utility of such a device
is (a) to extract light from optical fibers in a switchable and
channel sensitive manner; (b) to interconnect fibers to free space
detectors or lasers for testing or measuring the strength of
various wavelength channels or other purposes; or (c) to
interconnect light from fiber to fiber or waveguide plane to
waveguide plane in a complex switching network. Other uses would
also be apparent to those skilled in the art. With suitable grating
period, the integrated structure of FIGS. 4a, 4b could also couple
to space, but is more generally used to couple between waveguides
as previously discussed.
[0176] Attenuator
[0177] A further device consisting of an ESBG film on a
coupler-half is a fiber optic attenuator 40, FIG. 18. An attenuator
differs from a switch, which is optimized for two distinct states,
on and off, powered and unpowered. In contrast, the purpose of an
attenuator is to provide a long scale of voltage controlled loss
over a range of at least 30 dB (1000:1) for precise control of the
optical intensity propagating through the fiber. Such functions are
desired for example to equalize the strengths of signals from
different sources in a fiber optic network.
[0178] This can be constructed similarly to the ESBG film devices
previously described, except that a different fabrication scheme
for the ESBG film is provided. By producing a grating with very
small period .LAMBDA., substantially less than the approximately
0.5 .mu.m useful for Bragg reflection filtering at 1550 nm, an ESBG
film will produce a loss to the optical signal but will not be
highly wavelength selective. In the example of use at 1550 nm and
photosensitization of the H-PDLC for 488 nm, a very short period
grating of this type can be produced simply by increasing the half
angle between the argon laser or other exposing laser beams to be
substantially larger than for the prior devices, for example larger
than 30.degree.. Such a method produces a "subwavelength" grating,
i.e., one whose period is so short that instead of coupling forward
to backward modes, or guided to radiated modes, or guided to guided
modes, it simply acts as a quasi-homogeneous composite optical
medium whose role is now to provide electro-optic index control
over an unusually wide index range. In this case, the ESBG grating
serves primarily to form very small droplets, since the grating
period as such is largely irrelevant. The half angle .theta. used
for exposure will depend on the desired grating period .LAMBDA. and
can be determined from the period in ways known in the art (i.e.
sin .theta.=.lambda./2.LAMBDA. where .lambda. is the center
wavelength of the band to be attenuated and .LAMBDA. is the grating
period). Such subwavelength gratings may also be formed using a
master grating or binary phase mask technique previously described.
By virtue of the distribution of extremely small microdroplets
whose dimensions (on the order of 30-100 nm) and whose interplane
spacing (100-400 nm) are so small, the ESBG film is low in
scattering. The optical active property is then simply that the
average refractive index can be adjusted by an applied field.
[0179] In addition, by virtue of providing a very short period, the
polymerization strains that result in "football" shaped
microdroplets 26 are reduced, resulting in more nearly spherical
droplets. Since the sudden switching of the liquid crystal
directors in the microdroplets is strongly dependent on the
aspheric shape of the droplets, more nearly spherical droplets will
possess a reduced critical electric field, as may be inferred from
the published studies of Sutherland et al. This means that instead
of switching from one orientation to another at a well defined
critical voltage, the liquid crystals will rotate gradually as the
electric field is increased. This yields the long scale voltage
control of average refractive index required for operation as a
fiber optic attenuator.
[0180] A further requirement for good performance as an attenuator
is that the average refractive index of the ESBG film should
(through chemical formulation and subsequent holographic
polymerization), be designed to match the fiber optic cladding
index (typically, pure silica at 1.46) exactly in its powered
state, as can be accomplished by appropriate choice of liquid
crystal and polymer host species. This means that in the unpowered
state, the average index of such a film might, for example be
approximately 1.47, and the proximity of such a film in the
evanescent region of the polished optical fiber will result in a
substantial, wavelength independent loss. In the fully powered
state, the index is altered to match the silica cladding at 1.46,
in which case it effectively vanishes into the cladding from the
standpoint of the propagating mode, resulting in zero or very small
optical loss (attenuation). Between the unpowered and fully powered
state will be a long adjustment range of attenuation as a function
of applied electric field.
[0181] If the ESBG is index matched to the glass fiber as
described, the resulting attenuation will also be substantially
polarization independent. FIG. 19a shows the optical power
transmitted through the fiber as a function of electro-optically
altered ESBG index, for the two polarizations TE and TM. Note the
substantial equality. FIG. 19b shows the power transmitted as a
function of wavelength, for three levels of attenuation, indicating
substantial independence of wavelength.
[0182] Such a device constitutes a fiber optic attenuator whose
performance is independent of wavelength and polarization. Its
advantage over similar structures containing pure liquid crystal in
an overlay film is that the H-PDLC material has much lower optical
scattering and also does not require a separate orientation step,
since orientation within microdroplets is spontaneous. The
insertion loss of such a device will be very small.
[0183] Since the mechanism by which film or ESBG effects
attenuation is to selectively outcouple increasing percentages of
the signal on fiber 36 through film, such a film may also function
as a wavelength independent outcoupler. Further, rather than having
a homogeneous film, channels or stripes may be formed in film in
the direction parallel to fiber 36 which exhibit an index match
with the cladding which may be effective for reducing insertion
loss.
[0184] Channel Add/Drop Cross-Connects by Use of Grating Frustrated
or Grating Assisted Couplers
[0185] An additional class of devices can be constructed by placing
a second coupler half on top of the first, with an ESBG film
sandwiched in the middle. Such devices incorporate two optical
fibers 36, 36', with four optical ports 42A-42D. FIG. 20 shows one
variant of such a device. In this case, the ESBG film is exposed
and polymerized by means of a cover glass. Then the cover glass is
removed and replaced with an upper coupler-half 32' whose surface
has also been provided with an ITO or other transparent electrode
20. Alternatively, a liftoff (decal) deposition approach may be
used whereby the ESBG film is prepared and polymerized in a
laboratory fixture and then chemically released from its substrate
through floating in water or other methods, diced to size, and
bonded to the lower coupler-half 32.
[0186] In the device shown in FIG. 20, a multiplicity of wavelength
channels propagates in one single mode fiber into the device. The
device is designed, so that the fibers in the two coupler halves
are non-identical (see FIG. 16 of the '950 application), as may be
achieved for example by use of single mode fibers with slightly
different diameters or core indices, such that the propagation
constants for the various wavelength channels in the first fiber
(.beta..sub.i) differ from those (.beta.'.sub.i) in the second
fiber. In this case, in the absence of a grating film between them,
in spite of their proximity, the coupling will be low or
nonexistent because .beta..sub.i.noteq..beta.'.sub.i.
[0187] However if an ESBG grating is now provided between the two
nonidentical coupler halves and if for some one channel i the
grating period condition .LAMBDA. satisfies the condition 7 2 = i -
i ' Eq . ( 10 )
[0188] then the coupling will be grating-assisted and the i channel
will be coupled over to the second fiber, while all the other
channels will not be, and will continue to propagate in their
original fiber. Upon switching the ESBG off (i.e. applying power to
ESBG), this effect will be disabled, and all the signals will
propagate unaffected and uncoupled.
[0189] Thus, the grating assisted coupler using an ESBG between
nonidentical coupler halves has been described. Only one channel is
coupled from the first into the second fiber, the others
propagating unchanged, and in the powered state all channels
propagate unchanged.
[0190] 2.times.2 Space Switch
[0191] The devices described so far are wavelength selective. FIG.
21 shows a variant structure designed to minimize wavelength
dependence and to selectively couple light from a fiber 36 in
coupler-half 32 into a second identical fiber 36' in coupler-halve
32'. As shown in FIG. 21, this device differs from earlier
embodiments in that the two halves are bonded at an angle 2.theta.
to one another, with an ESBG film 12 sandwiched in between. If the
grating period .LAMBDA. satisfies the condition 8 sin = ( 2 n eff )
Eq . ( 11 )
[0192] then light will be coupled from the input port 42A of fiber
36 into the output port 42D of fiber 36' and from the input port
42B of fiber 36' to the output port 42C of fiber 36 using the
unpowered state of the grating, i.e. the light will be exchanged
from fiber to fiber. In its powered state, the periodic index
variation constituting the grating is turned off and the coupling
is disabled, keeping all signals in their original fibers. For
example, if .lambda.=1550 nm and .theta.=45.degree., .LAMBDA.=746
nm.
[0193] For this application, it is desirable to use a short
interaction length and a large liquid crystal loading in
formulating the H-PDLC so as to obtain large index modulation. In
essence, this is an embodiment of the 2.times.2 space switches
described in the '950 application, except that the fiber optic
coupler and overlay film methods are used in this embodiment.
[0194] FIGS. 22a and 22b describe still another form of a grating
assisted coupler. Two fibers 36, 36' are mounted on curved paths in
single silica block. On the polished surface, an ESBG grating 12
connects the two evanescent regions with a polymer guiding film.
This device is analogous to the "s" bend planar device described
earlier, except that the ESBG is now in a planar region, not a
confined channel waveguide. The grating is capable of coupling the
fiber mode to slab modes that propagate off axis, leading to
broadening of the drop bandwidth, and a spreading of the field
before being out-coupled by the second fiber. This results in
reduced efficiency and a somewhat degraded spectral response, as
illustrated in FIG. 22c. Such a response would however be
substantially improved if the ESBG film, rather than being a
two-dimensional planar layer, is etched to reduce it to a narrow
channel parallel to the fiber cores.
[0195] While the invention has been described above with respect to
a variety of embodiments and various processes have been described
both for fabricating individual devices and structures containing
multiple such devices, these various devices, structures and
methods have been provided by way of example only and it will be
apparent to those skilled in the art that numerous variations,
including ones discussed above on these devices, structures and
methods are possible while still practicing the teachings of the
invention, which is to be limited only by the following claims.
* * * * *